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