Xref: utzoo comp.sys.ibm.pc:20562 comp.misc:3918
Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!apple!voder!pyramid!ctnews!starfish!cdold
From: cdold@starfish.Convergent.COM (Clarence Dold)
Newsgroups: comp.sys.ibm.pc,comp.misc
Subject: Re: DB25 to DB9 Serial Problems
Summary: RS232 probing
Keywords: serial ports, diablo 1640, pinouts
Message-ID: <776@starfish.Convergent.COM>
Date: 25 Oct 88 17:02:56 GMT
References: <2521@silver.bacs.indiana.edu>
Organization: Convergent Technologies, San Jose, CA
Lines: 41

In article <2521@silver.bacs.indiana.edu>, regoli@silver.bacs.indiana.edu (michael regoli) writes:
> would some kind soul figure out why this cable situation doesn't work?
> i'm trying to convert a DB-25 to a DB-9 interface.
> [...] 
> what i've done is connected the following pins:
> 
>              DB-25     ------------->   DB-9
>              -----                      ----
>                1          (prot gnd)      ?
>                2          (TD)            3
>                3          (RD)            2
>                4          (RTS)           7
>                5          (CTS)           8
>                6          (DSR)           6
>                7          (sig gnd)       5
>                8          (DCD)           1
>               20          (DTR)           4          
>	[...] 
The pinout you have is correct for an adpater from the old cable to the new
9pin port.
You can verify which pins are being driven by the printer, which are expected
to be received, and which are empty with a DVM (high impedance voltmeter).
If pin 2 is really transmit data, measuring from pin 2, using pin 7 as
reference should should -8V when idle.  Other driven signals will show either
+8v or -8V.  This voltage might be different on different systems, but it 
should be a strong positive or negative.
Pins attached to receivers will should some slight voltage, probably around
0.5v on pin 3.
Pins that are unused should show 0V.
The only catch to this DVM trick is that some systems will send unused pins
to ground or +5 through a pullup or pulldown resistor.  I recall that pin
6, DSR had to be high on a Diablo, before it would accept data.  If this is 
pulled high inside the printer, then driven low by a PC signal, it could be
that eliminating a few pins from your adapter will make it work.  Try just
pins 2 and 7 to start, to see if you can get any data through.

-- 
---
Clarence A Dold - cdold@starfish.Convergent.COM		(408) 435-5274
		...pyramid!ctnews!professo!dold		MailStop 09-031
		P.O.Box 6685, San Jose, CA 95150-6685