Synchronous serial Re: E-Mail Formats RE: Future of cctalk/cctech

Paul Koning paulkoning at
Thu Jun 18 15:31:23 CDT 2020

> On Jun 18, 2020, at 3:26 PM, Ethan Dicks via cctalk <cctalk at> wrote:
> On Thu, Jun 18, 2020 at 3:08 PM Chuck Guzis via cctalk
> <cctalk at> wrote:
>> On 6/18/20 11:55 AM, Ethan Dicks via cctalk wrote:
>>> We used to run our sync serial stuff between 9600 and 56kbps, both our
>>> own Bisync products, and DDCMP over interfaces like the one that's
>>> part of the DMF32...
>> My recollection of the Bell 209 is that it supported a low-speed reverse
>> channel in addition to the FDX primary.
> I had to look that up.  Yes.  I see that in the spec, at *5*bps.

Wow, that's weird.

>> Did any DEC equipment ever take advantage of that?
> I've never encountered it before, so I cannot confirm.

Not that I know of.

I did see something vaguely similar.  Bell 202 modems are 1200 baud FSK, so on a voice channel they normally are 1200 bps half duplex.  They can also be hooked up to 4-wire fixed circuits.  But they have a reverse channel, good for 150 baud if I remember right.  PLATO used that in its original terminal connections, in a slightly strange way: 1260 bps data to the terminal, and 126 bps data from terminal to host.  The protocols are peculiar: terminal output is "synchronous", 21 bit frames at 60 frames per second, but each frame has a start bit (no stop bit).  Data to the host is asynchronous, 1 start, 1 stop bit, 10 data bits.  Since a 202 modem is just plain FSK, it doesn't matter that the data rate is not quite the standard 1200 bps.


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