[net.micro.mac] Serial Cable for Mac+: 8-pin to DB-25

gat@inuxd.UUCP (G Taylor) (03/21/86)

I have been trying to obtain a Mac+ 8-pin to DB-25 serial cable.
Since these seem to be hard to come by I decided to do surgery on 
an Apple System Peripheral-8 cable (the M0187 cable that has the 
8-pin connnectors on each end).  I'm not a hardware
type, but I can use a soldering iron and so off I went ....

Well, I almost got what I wanted but I hit a snag.  The following
recounts what I found.  Can anyone out there help me finish this
job???

Technical Note #65 (which was distributed on this net), indicates that
the M0187 cable is a null-modem cable (e.g. both the Mac+ and
Imagewriter II which use this cable are DTE) and indicates the
functions of each of the conductors in the cable.  Using that
information and a knowledge of RS232 I went to work.  First I cut the
cable in half and traced out the conductors to the connector pins.
Next I wired up the DB-25 connectors based on the Tech Note
information.  That almost worked but not quite.  The handshake signals
seem to be reversed from the Apple description.  The table below
indicates what I did and what Apple calls each function:

=================================================================
          M0187 Cable
8-Pin   Half-1  Half-2  25-Pin (EIA Fcn)    Apple Description
------  ------  ------  ----------------   ----------------------
 1       Red    White    8 (DCD)            Handshake Out
 2       White  Red      20&4 (DTR & RTS)   Handshake In
 3       Black  Blue     2 (TxD)            TxD-
 4       Yellow Yellow   7 (GND)            Ground
 5       Blue   Black    3 (RxD)            RxD-
 6       Green  Grey     no connection      TxD+
 7       Brown  Brown    no connection      No Connection
 8       Grey   Green    no connection      RxD+
=================================================================

 The first thing to note is that DCD (RS232 pin 8) is an output from
 a modem but the Tech Note identifies the signal I connected this to
 as an output from the Mac.  I tried it Apple's way, but the comm
 package I use got very confused trying to determine the modem's
 status.  When I switched this signal as shown in the table,
 everything seemed to work fine!

 NOW FOR MY PROBLEM:

 Although transmit and receive and the status signals are connected
 correctly, I get tremendous noise on the
 connection.  The problem isn't with the modem or the phone line since
 I completed the remainder of the cable to preserve the null-modem
 arrangement (I want to put the Imagewriter and modem on a switch box and
 switch between them).  When the Imagewriter is accessed through the
 "fixed" cable I get the same horrendous noise problems as I do with the
 modem.  SOMETHING ISN'T RIGHT HERE!  The Apple Tech Note indicates
 that the RxD+ line should be grounded "to emulate RS232."  I tried
 that; no luck.  I have tried RxD+ disconnected as in the
 table; no luck.  I even did the unthinkable and grounded the TxD+
 line; no luck.  I haven't tried grounding both RxD+ and TxD+ because
 I don't know what that might do electrically.

 If anybody understands what's going on and could help me out, I'd be
 extremely grateful.

 Glen A. Taylor
 AT&T (Consumer Products Division)
 Indianapolis, IN
 (317) 845-3709  inhp4!inuxd!gat