Bit banger /analog modem

[Allison]

>
>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