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From: joew@qat.UUCP
Newsgroups: comp.os.os9
Subject: Re: CoCo3 Serial Port
Message-ID: <2600004@qat>
Date: 6 Sep 90 16:18:00 GMT
References: <5444@lsuc.on.ca>
Lines: 46
Nf-ID: #R:lsuc.on.ca:5444:qat:2600004:000:2091
Nf-From: qat.UUCP!joew    Sep  6 11:18:00 1990


/* Written 10:10 pm  Sep  2, 1990 by lsuc.UUCP!jimomura in qat:comp.os.os9 */
/* ---------- "CoCo3 Serial Port" ---------- */

>      I've bbeen having a particularly annoying  problem for a
> long time now.  I'm using a DWP-210 printer on the "bit-banger"
> serial port for years.  A few months ago, after I added the diode
> by-pass ffor the CART line, the  bit-banger port started to become
> unreliable and then it went dead.  Now all I get is a #246 error
> (the serial port not ready).  I expected this to mean that the
> "printer ready" signal was not beinng detected on the CoCo3.  I
> tried substituting another old CoCco and the printer and cable
> were working properly.  So, not having adequate testing equipment,
> I started in changing thee parts that were related to this signal.
>
>      I've changed the jack, the SALT chip (I added a socket and
> new chip) and the resister and diode between the SALT and the PIA
> (in fact I changged both the diodes and resistors on the data-in
> circuit and Carrier Detect  lines), and I chaanged the PIA (I added
> a socket for the first PIA at that time).  None of this hhelped.
> I also added a switch in the diode by-pass line for the CART signal
> and that didn't help either.
>
>      There's another PIA "downstream" from the one I changed,
> and some other TTL chips.  It just doesn't seem likely to me that
> these would be the problem.  Does anybodyy have any advice or should
> I just keep on changing chips?
>
> Cheers! -- Jim O.
> --
> Jim Omura, 2A King George's Drive, Toronto, (416) 652-3880
> lsuc!jimomura
> Byte Information eXchange: jimomura


Another IC to check is IC15.  BUSY from the printer is "Serial Data In" to the
SALT IC.  This is converted to the TTL level signal SERINT.  This signal is
then sent to both the PIA (IC 4) and to IC15 (hex inverter).  The signal then
is inverted by IC15 to become the II2* input to the ACVC (IC 6).  So either
IC15 or IC6 could be damaged.

The other PIA (IC 5) handles the keyboard and some of the joystick information
and is not involved with serial port operation.


joe