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<H2><A NAME="cable"></A> <A NAME="s12">12.</A> <A HREF="Text-Terminal-HOWTO.html#toc12">Physical Connection </A></H2>
<H2><A NAME="ss12.1">12.1</A> <A HREF="Text-Terminal-HOWTO.html#toc12.1">Introduction</A>
</H2>
<P> A terminal may be connected to its host computer either by a
direct cable connection, via a modem, or via a terminal server. The
flow of data may be either a direct sequence of bytes (such as from a
serial port) or packets on a network (such as TCP/IP).</P>
<H2><A NAME="ss12.2">12.2</A> <A HREF="Text-Terminal-HOWTO.html#toc12.2">Multiport I/O Cards (Adapters)</A>
</H2>
<P> Additional serial cards may be purchased which have many serial
ports on them called "multiport boards". These boards are not covered
in this HOWTO but there is a list of some of them (with URLs) in the
Serial-HOWTO.</P>
<H2><A NAME="ss12.3">12.3</A> <A HREF="Text-Terminal-HOWTO.html#toc12.3">Direct Serial Cable Connection.</A>
</H2>
<P> The simplest way to connect a terminal to a host computer is via a
direct connection to a serial port on the computer. You may also use
some the info in this section for connecting one computer to another
(via the serial port). Most desktop PC's come with a serial port or
two, one of which may be used by a mouse. For the EIA-232 port, you
need a null modem cable (PC-to-PC cable) that crosses over the
transmit and receive wires. In ethernet terminology it would be
called a "crossover cable" (but the ethernet cable will not work for
the serial port). If you want hardware flow control, you will
probably use the DTR pin (or both the DTR and DSR pins).</P>
<P>Make sure you have the right kind of cable. A null modem cable
bought at a computer store may do it (if it's long enough), but it
probably will not work for hardware flow control. Such a cable may be
labeled as a serial printer cable. Only larger computer stores are
likely to stock such cables. A "modem cable" will not work since the
wires go straight thru (and don't cross over). See
<A HREF="#buy_or_make_cable">Buy or Make</A> your own cable. Make sure
you are connecting to your PC's serial port at the male DB25 or the
DB9, and not to your parallel port (female DB25).</P>
<H3>Pin numbering</H3>
<P>Pin numbers are often printed on the plastic right next to the
pins. You may need a bright light and/or a magnifying glass to read
them. Looking at the male pins of a DB connector with the wider row
up, the pin in the upper left is 1 (there is no pin 0). Then the next
pin in this row is 2, etc. At the end of this row is pin 5 or 13.
Then the next pin (6 or 14) is in the next row all the way to the left
and below pin 1. If you look at the female connector with the wider
row up, then pin 1 is in the upper right corner.</P>
<H3>Null Modem cable pin-out (3, 4, or 5 conductor)</H3>
<P> These 3 diagrams are for real text-terminals. But you could use
them to connect up 2 PCs if you substitute RTS for DTR and CTS for
DSR. (Don't use 4-conductors for PC-to-PC). For terminals, if you
only have DTR flow control (one-way) you may eliminate the RTS-to-DSR
wire. If you have no hardware flow control, then you may also
eliminate the CTS-to-DTR wire. Then if you have 2@ twisted pairs, you
may then use 2 wires for signal ground per
<A HREF="#twist_pair_kludge">A Kludge using Twisted-Pair Cable</A>. For a DB25 connector on
your PC, you need:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
PC male DB25 Terminal DB25
TxD Transmit Data 2 --> 3 RxD Receive Data
RxD Receive Data 3 &lt;-- 2 TxD Transmit Data
SG Signal Ground 7 --- 7 SG Signal Ground
CTS Clear To Send 5 &lt;--20 DTR Data Terminal Ready
RTS Request To Send 4 --> 6 DSR Data Set Ready
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>If you have a DB9 connector on your PC, try the following:
<BLOCKQUOTE><CODE>
<PRE>
PC DB9 Terminal DB25
RxD Receive Data 2 &lt;-- 2 TxD Transmit Data
TxD Transmit Data 3 --> 3 RxD Receive Data
SG Signal Ground 5 --- 7 SG Signal Ground
CTS Clear To Send 8 &lt;--20 DTR Data Terminal Ready
RTS Request To Send 7 --> 6 DSR Data Set Ready **
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>If you have a DB9 connector on both your serial port and terminal:
<BLOCKQUOTE><CODE>
<PRE>
PC DB9 Terminal DB9
RxD Receive Data 2 &lt;-- 3 TxD Transmit Data
TxD Transmit Data 3 --> 2 RxD Receive Data
SG Signal Ground 5 --- 5 SG Signal Ground
CTS Clear To Send 8 &lt;-- 4 DTR Data Terminal Ready
RTS Request To Send 7 --> 6 DSR Data Set Ready **
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>The above don't have modem control lines so be sure to give a "local"
option to getty (which is equivalent to "stty clocal"). Also if you
need hardware flow control it must be enabled at your computer (use a
-h flag with agetty) ( equivalent to "stty crtscts" ).</P>
<H3><A NAME="null_modem_pinout"></A> Standard Null Modem cable pin-out (7 conductor)</H3>
<P> The following 3 diagrams show full "standard" null modem cables.
One that you purchase may be wired this way. Another pinout is for 20
and 6 to cross over and to have 8 cross over to both 4 and 5. This
will not provide hardware flow control (RTS/CTS) for directly
connected computers. Both of the above will work for terminals using
software (Xon/Xoff) flow control (or no flow control). None of these
cables will work for terminal hardware flow control since most real
terminals support DTR or DTR/DSR flow control (handshaking) but Linux
doesn't yet (2000).</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
PC male DB25 Terminal DB25
DSR Data Set Ready 6 &lt;--|
DCD Carrier Detect 8 &lt;--|- 20 DTR Data Terminal Ready
TxD Transmit Data 2 ----> 3 RxD Receive Data
RxD Receive Data 3 &lt;---- 2 TxD Transmit Data
RTS Request To Send 4 ----> 5 CTS Clear To Send
CTS Clear To Send 5 &lt;---- 4 RTS Request To Send
SG Signal Ground 7 ----- 7 SG Signal Ground
DTR Data Terminal Ready 20 -|--> 8 DCD Carrier Detect
|--> 6 DSR Data Set Ready
</PRE>
</CODE></BLOCKQUOTE>
<A NAME="DB_pin-out"></A>
Alternatively, a full DB9-DB25 file-transfer (null-modem) cable (will
not work with terminal hardware handshaking; see above):</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
PC DB9 Terminal DB25
RxD Receive Data 2 &lt;---- 2 TxD Transmit Data
TxD Transmit Data 3 ----> 3 RxD Receive Data
|--> 6 DSR Data Set Ready
DTR Data Terminal Ready 4 -|--> 8 DCD Carrier Detect
SG Signal Ground 5 ----- 7 SG Signal Ground
DCD Carrier Detect 1 &lt;--|
DSR Data Set Ready 6 &lt;--|- 20 DTR Data Terminal Ready
RTS Request To Send 7 ----> 5 CTS Clear To Send
CTS Clear To Send 8 &lt;---- 4 RTS Request To Send
RI Ring Indicator 9 (not needed)
</PRE>
</CODE></BLOCKQUOTE>
(Yes, the pins 2 and 3 really do have opposite meanings for DB9 and
DB25 connectors!)</P>
<P>Here's how to connect two DB9's together (but DTR flow control will
not work):
<BLOCKQUOTE><CODE>
<PRE>
PC DB9 DB9
RxD Receive Data 2 &lt;----- 3 TxD Transmit Data
TxD Transmit Data 3 -----> 2 RxD Receive Data
|--> 6 DSR Data Set Ready
DTR Data Terminal Ready 4 --|--> 1 DCD Carrier Detect
GND Signal Ground 5 ------ 5 GND Signal Ground
DCD Carrier Detect 1 &lt;--|
DSR Data Set Ready 6 &lt;--|-- 4 DTR Data Terminal Ready
RTS Request To Send 7 -----> 8 CTS Clear To Send
CTS Clear To Send 8 &lt;----- 7 RTS Request To Send
RI Ring Indicator 9 (not used)
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>Using the above 2 connections provide full modem control signals and
seemingly allow one to set "stty -clocal". Then one must turn on the
terminal first (asserts DTR) before the port may be opened in a normal
manner by getty, etc. But there is likely to be trouble if you fail
to turn on the terminal first (see
<A HREF="Text-Terminal-HOWTO-19.html#rapid_respawn">Getty Respawning Too Rapidly</A>). For this reason one should use "stty
clocal" which is the default (ignores modem control lines) and the
additional wires in these cables then serve no useful purpose.</P>
<P>In olden days when it may not have been this easy to ignore modem
control signals etc, the following "trick" was done for cables that
lacked conductors for modem control: on your computer side
of the connector, connect RTS and CTS together, and also connect DSR,
DCD and DTR together. This way, when the computer needs a certain
handshaking signal to proceed, it will get it (falsely) from itself.</P>
<H3><A NAME="length_"></A> Overcoming length limitations </H3>
<P> A cable longer than a 50 feet or so may not work properly at high
speed. Much longer lengths sometimes work OK, especially if the speed
is low and/or the cable is a special low-capacitance type and/or the
electronics of the receiving end are extra sensitive. It is claimed
that under ideal conditions at 9600 baud, 1000 feet works OK. One way
to cover long distances is to install 2@ line drivers near each serial
port so as to convert unbalanced to balanced (and conversely) and then
use twisted pair cabling. But line drivers are expensive.</P>
<P>Another way to increase the distance is to try to cancel out much of
the magnetic field created by the currents in the transmit and receive
data wires: TxD and RxD. To do this, ground return lines, which have
current which is roughly equal (but in the opposite direction) are
placed next to the transmit and received wires. Twisted pair has
the best cancellation. Some DEC terminals have two signal ground
wires for this purpose. For example, one pair would be TxD and
SG(TxD) where SG is signal ground. If you use ribbon cable, insure
that the TxD and SG(TxD) wires are right next to each other.
Similarly for the RxD.</P>
<P>If there is only one signal ground wire provided by both the PC and
the terminal, it may be split into two wires in a twisted pair cable
for this purpose. You might think that return currents will be
equally split between the two signal ground wires. This would cancel
out only about half of the magnetic field. But it's better
cancellation than this because return current prefers the path of
least impedance. The return path of a data signal (such as TxD) has
the lowest impedance (due to lower inductance) if it flows back in the
same twisted pair. Although I've haven't seen any experimental test
results for this method, it should allow longer cable lengths.</P>
<H3>Hardware Flow Control cables</H3>
<P> If you expect to use hardware flow control (handshaking) you will
likely need to make up your own cable (or order one made). Of course,
if the connecters on the ends of a used cable remove, you might rewire
it. See
<A HREF="#db_conn_install">Installing DB Connectors</A>.
You will need to determine whether or not the terminal uses the
DTR pin for this, and if not, what pin (or pins) it uses. The set-up
menus may give you a clue on this since there may be an option for
enabling "DTR handshaking" (or flow control) which of course implies
that it uses the DTR pin. It may also use the DSR pin. See
<A HREF="Text-Terminal-HOWTO-11.html#hdw_flow_control">Hardware Flow Control</A> for a detailed
explanation of it. Older terminals may have no provision for hardware
flow control.</P>
<H3>Cable tips</H3>
<P> The normal "straight thru" cable will not work unless you are
using it as an extension cable in conjunction with either a null modem
(crossover or file-transfer) cable or a null modem adapter. Make sure
that the connectors on the cable ends will mate with the connectors on
the hardware. One may use telephone cable which is at least
4-conductor (and possibly twisted pair). Shielded, special
low-capacitance cable computer cable is best.</P>
<H3><A NAME="twist_pair_kludge"></A> A kludge using twisted-pair cable </H3>
<P> See also
<A HREF="#length_">Overcoming Length Limitations</A>.
Although none of the EIA-232 signals are balanced for twisted pair
one may attempt to use twisted-pair cable with it. Use one pair for
transmit and another for receive. To do this connect signal ground to
one wire in each of these 2 pair. Only part of the signal ground
current flows in the desired wire but it may help. Due to the lower
inductance of the twisted pair circuit (as compared to ground return
current by some other path) more return (ground) current will confine
itself to the desired twisted pair than one would expect from only
resistance calculations. This is especially true at higher
frequencies since inductive impedance increases with frequency. The
rectangular wave of the serial port contains high frequency harmonics.</P>
<H3>Cable grounding</H3>
<P> Pin 1 (of a DB25) should be chassis ground (also earth ground) but
on cheap serial ports it may not even be connected to anything. A
9-pin connector doesn't even have a chassis ground. The signal ground
is pin 7 and is usually grounded to chassis ground. This means that
part of the signal current will flow thru the ground wires of the
building wiring (undesirable). Cable shields are supposed to be only
grounded at one end of the cable, but it may be better to ground both
ends since it's better to have current in the shield than in the
building wiring ??</P>
<H2><A NAME="ss12.4">12.4</A> <A HREF="Text-Terminal-HOWTO.html#toc12.4">Modem Connection</A>
</H2>
<P> By using a terminal-modem combination (without a computer) one may
dial out to other computers. Up to the mid 1990s in the US, there
were many "bulletin boards" one could dial out to. Some even provided
connections to the Internet. But bulletin boards lost out in favor of
the Internet.</P>
<H3>Dialing out from a terminal</H3>
<P> Instead of connecting a terminal (or computer emulating a
terminal) directly to a host computer using a cable it may be
connected to the host via a telephone line (or dedicated private line)
with a modem at each end of the line. The terminal (or computer) will
usually dial out on a phone line to a host computer.</P>
<P>Most people use a PC and modem for dialing out. The PC could have a
terminal connected to a serial port and the person at the terminal
may dial out using the PC. Connecting a real terminal directly to an
external modem is more difficult since the real terminal isn't very
intelligent and doesn't give as much feedback to the user. For
dialing out, many terminals can store one or more telephone numbers as
messages which may be "set-up" into them and are sent out to the modem
by pressing certain function keys. Many modems can also store phone
numbers. The modem initiation sequence must precede the telephone
number. When the outgoing call is answered by another modem at the
other end of the phone line, the host computer on this modem may
run a getty program to enable you to log in.</P>
<H3>Terminal gets dialed into</H3>
<P> It's common for a computer running Linux to get dialed into. The
caller gets a login prompt and logs in. At first glance, it may seem
strange how a dumb terminal (not connected to any computer) could
accept an incoming call, but it can. One possible reason for doing
this is to save on phone bills where rates are not symmetric. Your
terminal needs to be set up for dial-in: Set the modem at your
terminal for automatic answer (Register S0 set to 2 will answer on the
2nd ring). You turn on the terminal and modem before you expect a
call and when the call comes in you get a login prompt and log in.</P>
<P>The host computer that dials out to your terminal needs to do
something quite unusual. As soon as your modem answers, it needs to
run login (getty). A host may do this by running the Linux program
"callback" sometimes named "cb". Callback is for having computer A
call computer B, and then B hangs up and calls A back. This is what
you want if you are using computer A to emulate a terminal. For the
case of a real terminal this may be too complex a task so the host may
utilize only the "back" part of the callback program. The setup file
for callback must be properly configured at the host. Callback makes
the call to the terminal and then has mgetty run a login on that port.
Mgetty by itself (as of early 1998) is only for dial-in calls but
there is work being done to incorporate callback features into it and
thus make it able to dial-out. As of early 1999 it didn't seem to have
been done.</P>
<H2><A NAME="ss12.5">12.5</A> <A HREF="Text-Terminal-HOWTO.html#toc12.5">Telnet and ssh</A>
</H2>
<P>Telnet is a program which lets a text terminal (or a PC console)
connect to a host computer over a network. No serial ports are used
for the telnet connection. Of course if you are sitting at a real
text terminal there is a serial connection to your own host. But when
you run telnet, your host connects to another host via serial-less
telnet.</P>
<P>Telnet uses tcp/ip packets over various networks: the Internet, LANs,
etc. You run telnet (as a client) and it connects to a telnet server
on another computer on a network. Then you get a login prompt and log
in just as if you were directly connected via a cable to a serial
port.</P>
<P>Ssh is "Secure Shell" and is like telnet. At one time it was much
more secure than the conventional telnet which sent passwords in the
clear (no encoding). But now (2006) there are packages "telnet-ssl"
which offer secure telnet. But since telnet was slow in introducing
security, ssh may have become more popular.</P>
<P>Telnet may put each character you type inside a packet and naturally
there's a lot of overhead with this. But there's an option for sending
one line at a time. To use telnet as a client, you need to have an
account on another computer on that runs a telnet server. Then you
give the telnet command and specify the address of the computer that
will accept a connection from you.</P>
<P>What kind of terminal does telnet emulate? It doesn't. Instead, it
connects you to a remote computer using whatever kind of terminal you
are currently using. If you're on a Linux console, it's a terminal of
type Linux. The remote computer needs to somehow find out what type
of terminal you're on so it can send the correct escape sequences.</P>
<P>So using telnet is much like connecting up a dumb terminal to a serial
port except that instead of a cable between you and the computer,
there is a stream of packets flowing over the Internet between you and
some distant computer.</P>
<H2><A NAME="ss12.6">12.6</A> <A HREF="Text-Terminal-HOWTO.html#toc12.6">Terminal Server Connection</A>
</H2>
<H3>What is a terminal server ?</H3>
<P> A terminal server is something that serves to connect a bunch of
terminals to a host computer(s) via a network. Today this server is
often located nearby or inside the host computer. If you directly
connect some terminals to a PC or connect them via dial-up modems thru
serial ports at each end, you don't need a terminal server.</P>
<P>But if the terminals are connected to the host over a network, then
you may need a terminal server to make the serial-to-network
conversions. This is useful for devices such as printers and
terminals that have no built-in network support. However the
definition of "terminal server" has broadened to the case where all
data flows entirely over a network (except of course within the
computer itself) and where no serial ports are involved. The term
"terminal" may include a thin client type terminal with a GUI. The
network usually uses tcp/ip and/or ppp but other protocols (including
protocol conversion) are sometimes supported.</P>
<P>One way to connect a "terminal" (your PC console) to a network is to
run telnet on your PC (assuming your PC has a network connection). At
one time, terminal servers were dedicated hardware which could be used
only as terminal servers. Today a PC can simultaneously serve as a
terminal server, thereby serving many terminals.</P>
<P>Today, most terminal servers serve thin client terminals rather than
text-terminals. The "Linux Terminal Server Project" is an example.
But it can also serve text terminals using telnet. Such a text-terminal
is likely to be just a PC monitor emulating a terminal of type
"Linux". The terminal server is just software running on the host
computer. Telnet server software is like a simple terminal server.</P>
<P>A host that only has directly connected terminals (or modem
connections without tcp/ip or ppp) is sometimes called a "terminal
server". Although it's doing the same job as a real terminal server,
it strictly speaking is not a terminal server. </P>
<H3>Evolution of the "terminal server"</H3>
<P>Originally a terminal server served real text-terminals via the
serial port. A server for real text-terminals would have a number of
serial ports. The user would log in to the server and then get
connected via tcp/ip, etc. to a host computer where s/he would login a
second time again. Sometimes the first login would be automatic, or
perhaps there would be a choice given the user as to what host
computer (or printer) to connect to (or what protocol to use).</P>
<P>The use of real text-terminals declined as the PC replaced mainframes.
But the PC could emulate a terminal (using say minicom (Linux) or
(hyper)terminal for MS). One could could then dial-out via a modem
to a bulletin-board or the like. There would be a bank of modems to
accept such calls and each modem would be connected to a serial port.
The serial port could either be on a multiport card or on a dedicated
terminal server. Note that in both the above cases there is no client
software. It's not a client-server model.</P>
<P>When the Internet became popular, one would run the PPP protocol on
the phone line and still go thru a modem and "terminal server" at the
ISP. This server would handle PPP and ultimately connect one to the
Internet. But the PC was no longer emulating a text terminal since
browser images were being displayed. Today with ISPs getting only
digital signals from the phone company, they don't need real modems
anymore. So what was once a "terminal server" evolved into a "remote
access server". It's infrequently been called a "digital terminal
server". Note that 56k modem service requires that an ISP have a
digital connection to the phone company.</P>
<P>With remote access servers, instead of many individual telephone line
cables connected to a terminal server, one now finds just a few cables
with many digitized telephone calls on each cable (multiplexed). The
multitude of connectors needed for large numbers of terminals or
modems is no longer present on a remote access server and thus the
successor to this type of terminal server can't readily serve
text-terminals anymore.</P>
<P>More recently with the advent of thin clients terminals, the term
"terminal server" was revived to apply to the hosts that served the
thin clients. Both MS Windows and Linux can serve thin clients.</P>
<H2><A NAME="ss12.7">12.7</A> <A HREF="Text-Terminal-HOWTO.html#toc12.7">Connector and Adapter Types</A>
</H2>
<P> A connector is more-or-less permanently attached to the end of a
cable or to a hardware unit. There are two basic types of connectors
used in serial communications: 1. DB types with pins (DB 9 or DB25) and 2.
modular telephone-style connectors.</P>
<P>An adapter looks about like a connector but it has two ends with pins.
It is just like a cable that is so short that there is no cable part
left at all --just different connectors on each end is all that
remains. The adapter just plugs in to two other connectors on each side
of it. It allows two incompatible connectors to mate with each other
by going in between them. Except that even for two connectors that
will mate with each other, an adapter may be used to connect the cable
conductors together in other than straight-thru. Obviously, one may
use a special cable (perhaps homemade) as a substitute for an adapter.</P>
<H3>Sex of connector/adapters</H3>
<P> Connectors (or one side of adapters) are either male or female.
The connectors that have pins are male and the ones that have sockets
(sometimes also called pins) are female. For modular connectors, the
ones with exposed contacts are plugs while the ones with internal
contacts (not easy to see) are jacks. Plugs are male; jacks are
female.</P>
<H3>Types of adapters</H3>
<P> There are three basic types of adapters: null modem, gender
changers and port adapters. Some adapters perform more than one of
these three functions.
<UL>
<LI> null modem adapter: Reroutes wires, like a null modem cable.</LI>
<LI> gender changer: Changes the sex of a cable end. Two connectors
of the same sex can now connect (mate) with each other.</LI>
<LI> port adapter: Goes from one type of connector to another (DB9
to DB 25, etc.)</LI>
</UL>
</P>
<H3><A NAME="db_conn"></A> DB connectors </H3>
<P> (For how to install a DB connector on the ends of a cable see
<A HREF="#db_conn_install">Installing DB Connectors</A>.) These come in
9 or 25 pins. The EIA-232 specs. call for 25 pins but since most of
these pins are not used on ordinary serial ports, 9 pins is
sufficient. See
<A HREF="#DB_pin-out">DB9-DB25</A> for the pin-out.
The pins are usually numbered if you look closely enough or use a
magnifying glass.</P>
<H3><A NAME="rj_conn"></A> RJ modular connectors </H3>
<P> RJ means Registered Jack. These look like modern telephone
connectors but are sometimes not compatible with telephone connectors.
See also
<A HREF="#rj_conn_install">Installing RJ Connectors</A>.
For use with serial ports they may be 6 or 8 conductor. A few are
10-conductor but may not officially belong to the RJ series.</P>
<H3>6-conductors: RJ11/14, RJ12, and MMJ</H3>
<P> RJ11 are all the same size but may have 2, 4, or 6 conductors.
If it has two conductors, it should be called a RJ11. If it has 4
conductors, some call it a RJ14. If it has 6 conductors, many call it
a RJ12 (but a RJ12 per the phone company has only 4 conductors).
Seems confusing but they are all the same size and differ mainly by
the number of conductor contacts present.</P>
<P>A look-alike (almost) is a MMJ connector (6-conductor) used on later
model VT (and other) terminals. It's sometimes referred to as a
DEC-423 or a DEC RJ11. MMJ has an offset tab and is not compatible
with RJ ones (unless the tab is cut off). However, some connectors
have been made that are compatible with both MMJ and the RJ ones.
Since MMJ connectors are both hard to find and may be expensive some
people have forced a RJ (6 conductor) to fit MMJ by filing off the
offset tab with a file.</P>
<P>The MMJ (DEC) pinout is: 1-DTR, 2-TxD, 3-TxD_Gnd, 4-RxD_Gnd, 5-RxD,
6-DSR. Cyclades Cyclom-8Ys RJ12 has: 1-DTR, 2-TxD, 3-Gnd, 4-CTS,
5-RxD, 6-DCD. Specialix IO8+ has: 1-DCD, 2-RxD, 3-DTR/RTS, 4-Gnd,
5-TxD, 6-CTS. The pins of the RJ (and MMJ) are numbered similar to
the RJ45.</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
Plug Jack (or socket)
(Looking at the end (Looking at the cavity
end of a cable) in a wall or PC back)
.________. .________.
| 654321 | | 123456 |
|__. .__| |__. .__|
|__| |__|
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>A standard MMJ file-transfer (null-modem) cable has a MMJ connector at
each end. It connects to the PC using a MMJ-to-DB adapter. This
adapter plugs into a DB (say 25 pin) connector on the back of the PC
and the MMJ connecter plugs into it. If you don't have such an
adapter, you can make a custom cable with a MMJ (or filed RJ)
connector on one end and a DB connector on the other end.</P>
<P>The standard file-transfer (null-modem) cable with two MMJ (or
RJ11/14) connectors will connect: 1-6, 2-5, and 3-4. Note that such a
cable supports DTR/DSR flow control which is not supported (yet) by
Linux. Making up your own standard 6-conductor file-transfer cable is
very simple if you understand that the ordinary 4-conductor telephone
cable from the wall to your telephone, used in hundreds of millions of
homes, is also a file-transfer cable. Find one and wire your cable the
same way.</P>
<P>If you lay such a cable flat on the floor (with no twists) you will
note that both plugs on the ends have their gold contacts facing up
(or both facing down). Although it's symmetrical, it is also
file-transfer if you think about it a bit. One may put a few such
cables together with inline couplers and everything works OK because
each inline coupler is also a file-transfer (null-modem) adapter. Two
file-transfer cables in series result in a straight-thru connection.</P>
<P>Here's a custom cable diagram (by Mark Gleaves) for connecting MMJ to
a 9-pin serial port using RTS/CTS flow control:
<BLOCKQUOTE><CODE>
<PRE>
DEC MMJ Linux PC DB9
Pin Signal Signal Pin
=== ====== ====== ===
1 DTR -----------------------|---> DSR 6
|---> CTS 8
2 TxD ---------------------------> RxD 2
3 SG (TxD)--------------------|--- SG 5
4 SG (RxD)--------------------|
5 RxD &lt;--------------------------- TxD 3
6 DSR &lt;-----------------------|--- RTS 7
|--> DTR 4
|--> CD 1
(no connection) RI 9
</PRE>
</CODE></BLOCKQUOTE>
</P>
<H3>8-conductors and 10-conductors</H3>
<P> RJ45 and RJ48 are 8-conductor modular telephone plugs.
There exists some 10-conductor connectors which are allegedly wider
and will not mate with the 8-conductor ones. People have called the
10-conductor ones RJ45 and/or RJ48 but this may be incorrect. These
connectors are used for both flat telephone cable and round twisted
pair cable. The cable end of the connector may be different for round
and flat cable. RJ48 has an extra tab so that a RJ48 plug will not
push into a RJ45 jack (but a RJ45 plug will mate with a RJ48 jack).
They're used on some multiport serial cards and networks. Heres the
pin numbers for an 8-conductor:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
Plug Jack (or socket)
(Looking at the end (Looking at the cavity
end of a cable) in a wall)
.__________. .__________.
| 87654321 | | 12345678 |
|__. .__| |__. .__|
|____| |____|
</PRE>
</CODE></BLOCKQUOTE>
</P>
<H2><A NAME="ss12.8">12.8</A> <A HREF="Text-Terminal-HOWTO.html#toc12.8">Making or Modifying a Cable</A>
</H2>
<H3><A NAME="buy_or_make_cable"></A> Buy or make ? </H3>
<P> You may try to buy a short, null modem cable. Just a "modem
cable" will not work. Null modem cables were once labeled as "serial
printer" cables but serial printers are not popular today (2004).
They also may be labeled as "file-transfer" cables or "PC-to-PC"
cables. Hardware flow control will work on them for PC-to-PC
connections (terminal emulation) but not for the DTR flow control used
by most real text terminals. If Linux supported DTR flow control,
real terminals would work OK . Make sure the connectors on the cable
ends will fit the connectors on your computer and terminal.</P>
<P>But if you need longer cables to connect up terminals or need hardware
flow control, how do you get the right cables? The right ready-made
cables may be difficult to find (you might find them by searching the
Internet), especially if you want to use a minimum (say 4) of
conductors. One option is to get them custom made, which is likely to
be fairly expensive although you might find someone to make them at
prices not too much higher than ready-made cable (I did).</P>
<P>A low-cost alternative is to buy used cables (if you can find them).
If you get a used terminal, ask if they have a cable for it. Another
alternative is to make your own. Even if you get used cables, they
may need some changes to the pin wiring. In either case, this may
require special tools. Most connectors that come with short cables
are permanently molded to the cable and can't be rewired but most
custom-made and homemade cables have connectors that can be rewired.
One advantage of making your own cable is that the skills you learn
will come in handy if a cable breaks (or goes bad) or if you need to
make up another cable in a hurry.</P>
<H3>Pin numbers of 9 and 25 pin connectors</H3>
<P> The pin numbers are often engraved in the plastic of the
connector but you may need a magnifying glass to read them.
Note DCD is sometimes labeled CD. The numbering of the pins on a
female connector is read from right to left, starting with 1 in the
upper right corner (instead of 1 in the upper left corner for the male
connector as shown below). --> direction is out of PC.</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
___________ ________________________________________
\1 2 3 4 5/ Looking at pins \1 2 3 4 5 6 7 8 9 10 11 12 13/
\6 7 8 9/ on male connector \14 15 16 17 18 19 20 21 22 23 24 25/
------ -----------------------------------
</PRE>
</CODE></BLOCKQUOTE>
</P>
<H3><A NAME="db_conn_install"></A> Installing DB connectors on cable ends </H3>
<P> See
<A HREF="#db_conn">DB Connectors</A> for a brief description
of them. Unfortunately, most cables one purchases today have molded
connectors on each end and can't be modified. Others have connectors
which unscrew and can be rewired. If you are making up cable or
modifying an existing one then you need to know about pins. There are
two types: soldered and crimped.</P>
<P>The crimped pins require a special crimping tool and also need an
"insertion/extraction" tool. But once you have these tools, making up
and modifying cable may be faster than soldering. If you are
connecting two wires to one pin (also needed if you want to jumper one
connected pin to another pin) then soldering is faster (for these
pins). This is because the crimped pins can only take one wire each
while the soldered ones can accept more than one wire per pin.</P>
<P>To insert crimped pins just push them in by hand or with the insertion
tool. Using the tool for either insertion of removal first requires
putting the tool tip around the wire. The tool tip should completely
encircle the wire at the back of the pin.</P>
<P>Removing a pin with this tool is a little tricky. These directions
can be best understood if you have both the tool and wires in front of
you as you read this. With the tool tip around the wire insert the
tool as far as it will go into the hole (about 1 1/2 cm. Some tools
have a mark (such as a tiny hole) on them to indicate how far to
insert it. The tool tip should have a tapered gap so that you may get
the tip around the wire by starting it in where the gap is wider than
the wire. The tool may have 2 tips. The one that is the most
difficult to get around the wire is also the one that removes the wire
the easiest since it almost completely envelops the wire.</P>
<P>With the tip properly inserted pull on both the tool and the wire
with a gentle pull. If it doesn't come out, the tool was likely not
inserted correctly so either push it in more or twist it to a
different position (or both). Perhaps you should have used another
tip that fits tighter around the pin. Using this tool, one may
readily convert a straight-thru cable to a file-transfer (null-modem)
cable, etc.</P>
<P>There can be problems using the "insertion/extraction" tool. If the
tools will not insert on the back of the pin, it could be that the pin
was not neatly crimped to the wire and is sort of square where it
should be round, etc. If a pin starts to come out but will not pull
out all the way, the pin may be bent. Look at it under a magnifying
glass. Straightening a pin with needle-nose pliers may damage the
gold plating but you may have to straighten it to remove it.
Sometimes a stuck pin may be pushed out with a thick screwdriver blade
tip (or the like) but if you push too hard you may gouge the plastic
hole or bend the pin:.</P>
<P>Don't try soldering unless you know what you're doing or have read
about how to do it.</P>
<H3><A NAME="rj_conn_install"></A> Installing RJ connectors </H3>
<P> These are telephone modular connecters one type of which is used
for most ordinary telephones. But there are many different types (see
<A HREF="#rj_conn">RJ Modular Connectors</A>).</P>
<P>These are not easy to reuse. You might be able to pull the wires out,
push in something wedged that would lift up the gold-colored contacts
and reuse the connector. There are special crimping tools used to
install them; a different tool for each type.</P>
<P>If you don't have a crimping tool, installation is still possible (but
difficult) using a small screwdriver (and possibly a hammer). Push in
the cable wires and then push each gold-colored contact down hard with
a small screwdriver that will just fit between the insulating ridges
between the contacts. You may damage it if you fail to use a
screwdriver with a head almost the same thickness as the contacts or
if the screwdriver slips off the contact as you are pushing it down.
You may also use a small hammer to pound on the screwdriver (push
first by hand).</P>
<P>Be sure to not hurt the "remove lever" on the connecter when you push
in the contacts. Don't just set it down on a table and push in the
contacts. Instead, put a shim (about 1 mm thick) that fits snugly
in the crevice between the lever and the body. For such a shim you
may use thick cardboard, several calling cards, or wood. Since the
bottom of the connector (that you will put on the table) isn't level
(due to the "remove lever), make sure that the table top has something
a little soft on it (like a sheet of cardboard) to help support the
non-level connector. Even better would be to put another 1mm shim
under the first 6mm of the connector, supporting it just under where
you see the contacts. A soft tabletop wouldn't hurt either. Another
method (I've never done this) is to hold the connector in a vice but
be careful not to break the connector.</P>
<P>As compared to using a crimping tool, installing it per above takes a
lot longer and is much more prone to errors and failure but it's sometimes
more expedient and a lot cheaper than buying a special tool if you
only have one or two connectors to install.</P>
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