[Coco] [Color Computer] Coco serial cable

[Aaron Wolfe]

On Fri, Aug 28, 2009 at 4:00 PM, Willard Goosey<goosey at virgo.sdc.org> wrote:
> On Fri, Aug 28, 2009 at 01:18:43AM -0400, Aaron Wolfe wrote:
>
>> using pin 11 for flow control isn't that strange on a serial printer,
>> there are several models that do this.
>
> Huh.  I've never (successfully) used a non-Tandy serial printer with
> my CoCo.
>
>>  pin 18 I have never seen used on a printer, but 19 is sometimes,
>> and especially with it tied to pin 11 i wonder if the op just
>> miscounted.
>
> Interesting.  Nothing in any of the various docs I've got around say
> anything about those lines.  Cool, I just learned something!
>

Pin 11 is the first "unassigned" pin in EIA standard rs-232-c.
perhaps this is why several companies used it for things a bit outside
of the rs-232 standard.   Printers from several manufacturers (GE,
Texas Instruments, DEC, Tektronix, etc)  used pin 11 for "secondary
request to send".   Others used it for Busy, Ready, Alarm type
signals: mostly some sort of flow control.

Pin 19 is defined in the standard as secondary request to send, which
makes you wonder why so many used pin 11 for this.  However, some of
those same devices did use pin 19 for 2nd clear to send.  Maybe there
was some confusion when the standards were being created, or maybe 2nd
rts/cts is "seen" from the opposite end?  I really don't know.  The
important thing is that it isn't uncommon to see pin 11 tied to 19 to
null out the secondary rts/cts just like you see pin 4 and 5 tied to
null rts/cts.

Pin 18 is weird though. I really can't find *anything* async that used
18, even among the models that used 11 for non standard stuff.  If you
ever do find out what that cable was for, I'd be interested to know
who used pin 18.

-Aaron