X-Modem Cables

The Problem: Want to connect 2 PCs via serial connection and don't know how the wiring of the connection cable goes.

The Solution: Follow the instructions below and built your own cable !

You need:
  • a soldering iron (a good one - as usual)
  • a multimeter with Ohms-range or continuity checker
  • 2 Sub-D female plugs (either 9 or 25 pin - see below)
  • some serial cable (see below too)
  • some self-confidence (like always)

PCs may have two different serial ports:

  • The standard 25-pin Sub-D port (2 rows of pins)
  • The AT-style 9-pin Sub-D port (2 rows of pins)
To cover all standard combinations I would recommend to build a cable with one 9-pin female plug and a 25-pin female plug on the opposite side and buy one 9-25 pin converter (9 pin female to 25-pin male) and one 25-9 pin converter (25-pin female to 9-pin male). This sounds a bit irrational, but if you see the cables and converter adapters it will come clear.

Combinations:


PC1 has                                                     PC2 has
-------------------------------------------------------------------

25-p |- ]---[25-p f - 9-p f]---[ -|----[9-p m/ 25-p f]----[ -| 25-p
                 -cable-              -converter 1-

25-p |- ]----------------[25-p f - 9-p f]---  ------------[ -|  9-p
                              -cable-

9-p  |- ]---[9-p f/ 25-p m]--|- ]-- [25-p f - 9-p f]------[ -|  9-p
              -converter 2-              -cable-


      [Table 1: Possible combinations with 2 converters]

The only combination not covered by this cable and converters is the use of male plugs as serial connector - which is not standard anyway.

Now we need an appropriate cable.

The absolute minimal requirement is a serial cable with 3 wires. I call it VSSC - Very Simple Serial Cable.
The disadvantage of this cable: some communication programs don't like it and don't work with it. It is only good for relatively slow serial data-transfer and -if possible- set the communication handshake to XON/XOFF, where the data transfer is controlled by the software.

With this cable only the RXD- and TXD-lines are crossed, while the protocol lines are tied to each other on either side - and generate a 'dummy handshake'.

Here's how the VSSC is wired: 


9-pin  25-pin  Signal              to           Signal   25-pin  9-pin

  2      3      RXD --------------\ /---------- RXD      3       2
                                   X
  3      2      TXD --------------/ \---------- TXD      2       3

  5      7      GND --------------------------- GND      7       5

  1      8      DCD --+                     +-- DCD      8       1
                      |                     |
  6      6      DSR --+                     +-- DSR      6       6
                      |                     |
  4     20      DTR --+                     +-- DTR      20      4
                      |                     |
  7      4      RTS --+                     +-- RTS      4       7
                      |                     |
  8      5      CTS --+                     +-- CTS      5       8 

     
             [Fig. 1: VSSC - 3 lines, all loop-backed]
Usually a Crossed Cable needs some more lines for the hardware-handshake. If you want LapLink or Interlink running on a serial cable you need a cable with some more wires - 7 to be precise.

The 'real' X-Modem cable looks like that:


9-pin  25-pin  Signal              to           Signal   25-pin  9-pin

  2      3      RXD --------------\ /---------- RXD      3       2
                                   X
  3      2      TXD --------------/ \---------- TXD      2       3

  5      7      GND --------------------------- GND      7       5

  1      8      DCD --+                     +-- DCD      8       1
                      |                     |
  6      6      DSR --+-----------\ /-------+-- DSR      6       6
                                   X
  4     20      DTR --------------/ \---------- DTR      20      4
                     
  7      4      RTS --------------\ /---------- RTS      4       7
                                   X
  8      5      CTS --------------/ \---------- CTS      5       8 

     
                   [Fig. 2: Normal X-Modem cable]

What about LapLink ? Do they use the same wirings ?

From the principle: Yes. However the original LapLink serial cable has a slightly different wiring. It goes like that:


9-pin  25-pin  Signal              to           Signal   25-pin  9-pin

  2      3      RXD --------------\ /---------- RXD      3       2
                                   X
  3      2      TXD --------------/ \---------- TXD      2       3

  5      7      GND --------------------------- GND      7       5

  1      8      DCD                             DCD      8       1
                                             
  6      6      DSR --------------\ /---------- DSR      6       6
                                   X
  4     20      DTR --------------/ \---------- DTR      20      4
                     
  7      4      RTS --------------\ /---------- RTS      4       7
                                   X
  8      5      CTS --------------/ \---------- CTS      5       8 

     
          [Fig. 3: The serial X-Modem  cable for LapLink]
You see: they do not use the "blind" DCD (Data Carrier Detected) loopback from DSR (Data Set Ready). Though it won't disturb the function of LapLink or any other serial product of that kind (like Norton Link, IBM Interlink or PC-Tools Link) if the DCD-DSR loopback is there. At least I haven't found any problem since I have only one common X-Modem cable for all my purposes.

Just for completeness: The parallel Laplink cable 


25 pin male                                   25 pin male

   2 --------------------------------------------- 15

   3 --------------------------------------------- 13

   4 --------------------------------------------- 12

   5 --------------------------------------------- 10

   6 --------------------------------------------- 11

  10 --------------------------------------------- 5

  11 --------------------------------------------- 6

  12 --------------------------------------------- 4

  13 --------------------------------------------- 3

  15 --------------------------------------------- 2


       [Fig. 3: The parallel cable for LapLink]

In either case it pays to buy a high quality cable and solder the outside shielding to the metal frame of the plug.

That's all !

Feel free to report experiences.

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© 1997 by Peter H. Wendt / pw-software production