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20.006 Does anyone have directions for making a "NULL Modem" cable?




Description

This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.

20.006 Does anyone have directions for making a "NULL Modem" cable?

         A typical NULL modem is two Dsub 25-pin female sockets (call them "A" and
"B") wired back-to-back as follows ...

 Socket-A  Socket-B
 
    2      -> 3
    3      -> 2
    4      -> 5
    5      -> 4
    6 & 8  -> 20
    7      -> 7
   20      -> 6 & 8
Note: 6 & 8 are connected at each socket. (That is, if you are making a cable instead of using back-to-back sockets, you do not want to run 6 and 8 separately to pin 20 on the other socket. For back-to-back sockets, it's okay to run separate short leads if you like.)
 A Dsub-25 female connector viewed from the front
  ,----------------------------------------.
  \ 13 12 11 10 09 08 07 06 05 04 03 02 01 /
   \  25 24 23 22 21 20 19 18 17 16 15 14 /
    `------------------------------------'
If you can find a couple old-style Dsub-25 plug casings, the sockets can be mounted and the casings can be glue-gunned together to make a nice compact unit. For Apple2-PC (or Mac, etc.) transfers, you plug the modem cable from the Apple II into one side of the NULL modem and the modem cable from the PC into the other side. If the PC has a 9-pin serial port connector, use a 9-to- 25 adaptor cable to connect to the NULL modem. Similarly,for Mac users, if the Mac cable has a special connector, use a converter to a male Dsub-25M connector for plugging into the NULL modem. Dedicated IIgs NULL modem cable The usual IIgs NULL modem cable consists of a High-Speed IIgs modem cable connected to a NULL modem adapter which connects to a modem cable coming from the PC. If you want to 'roll you own' single piece IIgs --> PC cable for NULL modem connections, here is the pin-out info:
 IIgs            PC  Female Dsub
Mini-Din 8   Dsub-25F  or  Dsub-9F   Dsub Signal
 
  3           3             2         RXD
  5           2             3         TXD
  4,8         7             5         GND
  2           4             7         RTS
  1           5,6,8         1,6,8     CTS, DSR, DCD
  7           20            4         DTR
----------------------------

 

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