TECHNICAL BULLETIN #87 - Rev 1.10 (9/03/96)
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TITLE : About Null Modem Cables
PRODUCT : Procomm Plus
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WHY USE A NULL MODEM CABLE?
Computers can communicate over both modem and direct cable connections. When
connecting two adjacent computers, you may use either of these methods.
Although you can connect your PC to a modem with a normal serial cable, you
must connect two PC's with a modified serial cable called a null modem cable.
Instead of using two modems and two phone lines, you can simply connect a null
modem cable between the serial ports of the two computers for a much easier
and less expensive way to transfer files between adjacent PC's.
WHERE CAN I BUY A NULL MODEM CABLE?
Your local computer store will most likely carry null modem cables. If you
have an extra serial cable around, you can buy a null modem adapter. Adapters
are usually less expensive and allow you to get the most out of what you
already have. If you need to buy a null modem cable, be sure to get one that
is both long enough and has the proper size connectors on it. If need be, you
can buy a 25 to 9 pin (or 9 to 25 pin) converter to arrive at the proper
connector size.
HOW DO I MAKE A NULL MODEM CABLE?
A null modem is cable is not much different from an average serial cable.
Null modem cables have simply been rewired to simulate a modem connection. To
create a null modem cable you will need one the following:
1. A length of shielded nine-conductor cable and two female connectors of the
appropriate size, or
2. A normal serial cable and a gender changer. You will take apart and
rewire the gender changer; or
3. A serial cable with two female connectors. You will disassemble the
connector housings and rewire the connectors.
You will need a soldering iron and some solder to create a null modem cable.
Whichever method you use, make the appropriate connections using one of the
following diagrams. See the section following the diagrams ("How to Read the
Diagrams") for an explanation of the diagram codes.
Guide for a 25 pin to 25 pin null modem cable:
FG 1-----------------------1 FG
TD 2----------\ /----------2 TD (pin 2 is tied to pin 3)
RD 3----------/ \----------3 RD (pin 3 is tied to pin 2)
RTS 4----------\ /----------4 RTS (pin 4 is tied to pin 5)
CTS 5----------/ \----------5 CTS (pin 5 is tied to pin 4)
SG 7-----------------------7 SG
DSR 6--------|\ /|--------6 DSR (pin 6 is tied to pin 8
DCD 8--------| \ / |--------8 DCD on both sides;
DTR 20---------/ \----------20 DTR both are tied to pin 20)
Guide for a 25 pin to 9 pin null modem cable:
25 Pin Side 9 Pin Side
TD 2-----------------------2 RD
RD 3-----------------------3 TD
RTS 4----------\ /----------7 RTS (pin 4 is tied to pin 8)
CTS 5----------/ \----------8 CTS (pin 5 is tied to pin 7)
SG 7-----------------------5 SG
DSR 6--------|\ /|--------1 DCD (pin 6 is tied to pin 8;
DCD 8--------| \ / |--------6 DSR pin 1 is tied to pin 6;
DTR 20---------/ \----------4 DTR both are tied to pins 20 & 4)
FG 1-----| |-----9 RI (pins 1 and 9 are terminated)
Guide for a 9 pin to 9 pin null modem cable
RD 2----------\ /----------2 RD (pin 2 is tied to pin 3)
TD 3----------/ \----------3 TD (pin 3 is tied to pin 2)
DTR 4--------|\ /|--------4 DTR (pin 4 is tied to pin 1
DCD 1--------| \ / |--------1 DCD on both sides;
DSR 6----------/ \----------6 DSR both are tied to pin 6)
SG 5-----------------------5 SG
RTS 7----------\ /----------7 RTS (pin 7 is tied to pin 8)
CTS 8----------/ \----------8 CTS (pin 8 is tied to pin 7)
RI 9-----| |-----9 RI (pins 9 and 9 are terminated)
HOW TO READ THE DIAGRAMS
The diagrams follow the pattern: abbreviation of line function, pin number,
connection graphic, pin number of other connector, abbreviation of other
connector's line function. For example:
SG 5-----------------------5 SG
This diagram shows that you are to connect pin 5 on the left side of the cable
directly to pin 5 on the right side of the cable. The SG denotes the these
are Signal Ground connections. You will find a complete explanation of all
abbreviations in the glossary below.
In this example:
RTS 7----------\ /----------7 RTS
CTS 8----------/ \----------8 CTS
you are instructed to connect pin 7 on the left side to pin 8 on the right
side. Pin 8 on the left side is connected to pin 7 on the right side. The
fact that the lines cross does NOT mean that these two wires should be
connected to each other. These are two totally independent signal paths. An
example where you would connect wires together is diagrammed as follows:
DSR 6---------|\ /|--------1 DCD
DCD 8---------| \ / |--------6 DSR
DTR 20----------/ \----------4 DTR
In this example you are to connect pins 6 and 8 on the left side to each other
and then connect them to pin 4 on the right side. You then connect pins 1 and
6 on the right side to each other and connect them to pin 20 on the left side.
Lastly, the symbol -----| |----- indicates a line which is not
connected to anything. It remains unused.
GLOSSARY
CTS.....Clear To Send. The CTS line is asserted by the PC (as DCE device)
when it is ready to receive data.
DCD.....Data Carrier Detect. The DCD line is asserted when the data link is
established.
DCE.....Data Communications Equipment. Refers to the modem in a computer to
modem setup.
DSR.....Data Set Ready. The DSR line is asserted by the DCE when it is ready
to communicate with the DTE.
DTE.....Data Terminal Equipment. Refers to the computer in a computer to
modem setup.
DTR.....Data Terminal Ready. The DTR line is asserted by the DTE when it is
ready to communicate with the DCE.
FG......Field Ground. A protective line used to ground the DCE.
RD......Receive Data. The RD line is used by the DCE to send data to the DTE.
RI......Ring Indicator. The RI line is asserted by the DCE when a ring is
detected.
RTS.....Request To Send. The RTS line is asserted by the DTE when it wants
to transmit data to the DCE.
SG......Signal Ground. The common return (and voltage baseline) for the
various signal lines.
TD......Transmit Data. The TD line is used by the DTE to send data to the
DCE.
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Copyright (c) 1996 Quarterdeck All rights reserved.
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