Bit banger /analog modem - test-drb@ccmp.vtda.org

20 Aug 2005

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Subject: Re: Bit banger /analog modem
   From: Ethan Dicks <ethan.dicks at gmail.com>
   Date: Sat, 20 Aug 2005 23:08:03 -0400
     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>

On 8/17/05, infomagic <infomagic at localisp.com> wrote:
  Tony wrote:
  have never seen a bit-banged RS232 (or current loop) port on a
 minicomputer. Anyone know of one?

 On the RCA 1802, there was an output line named "Q", and there were two
instructions that directly set and reset the line. I believe the CPU had a built-in
D-flip-flop for keeping it stable.

 It's my understanding that this line was often used to drive serial output. You might
be able to locate software for any of the ELF computers that would show this use. I'll
see what I can dig up... 
Except that the 1802 is most decidedly a microprocessor, not a minicomputer.

I am unaware of any mini that did bit-banged serial.  There were some
DEC console ports that impacted performance (11/750, 82x0/83x0), but
they were a hardware implementation AFAIK.  Perhaps HP or DG might
have an example?

-ethan 
VAX had a bit of effort doing this as realtime IO was not their forte.
PDP-11s however could easily do it.

There is a sample of code in the PDP-8 programmers handbook for doing 
bit banging on an -8.  While many machines used hardware to do this
it was enough hardware (easily done with out a uart) to warrent another
board and represents cost. The trade was using a little of the CPU cycles
to dothe IO in software.  For system near the bottom dollar wise this 
can save a few bucks. The real question is is it worth 1-10% of the 
cpu cycles for IO, and how much IO is needed?

Allison