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