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

[Ethan Dicks]

On Thu, Jun 18, 2020 at 6:08 PM Peter Coghlan via cctalk
<cctalk at classiccmp.org> wrote:
> Ethan Dicks wrote:
> > As for the clocking, yes, a modem or modem eliminator provides the
> > baud rate clocking on pins 15 and 17.  You could use any one of a
> > number of baud rate generators...

> I can rustle up +/-12V with a bench supply or two but I don't have a
> 1488 handy.  I should be able to borrow a MAX232 from something though.

I have an abundance of 1488/1489 chips, but I used to make comms
devices.  I only have a few MAX232 devices not already baked into
projects.

> I don't have any baud rate generators lying around either.  How about a
> 555 generating square waves round about 10kHz for something approximating
> 9600 bps?  Does it have to be spot on a "valid" rate or will anything
> "close" do as long as it is the same at both ends?

It does need to be the same on both ends, and, yes, a 555 should work,
but it's unlikely to be within 1% of a "real" rate.

> To be absolutely clear, do I have to drive pins 15 and 17 going to both
> interfaces ie four loads on the driver in total?

Yes.  Pins 15 and 17 on both ends.  4 loads.

You can do asymmetric speeds, but the TX for one end has to match the
RX for the other end.  I see some discussion of that in this thread,
but in my experience, we always had same-room direct connect lines, or
dial-up lines over POTS and modem speeds like 1200 bps or 2400 bps
because we didn't want to pay for leased or conditioned lines.

Every connection I ever set up was symmetric.  99.9% were 56kbps or slower.

-ethan