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

Dave Wade dave.g4ugm at
Fri Jun 19 03:25:46 CDT 2020

> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of Peter Coghlan
> via cctalk
> Sent: 18 June 2020 23:11
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at>
> Subject: Re: Synchronous serial Re: E-Mail Formats RE: Future of
> cctalk/cctech
> Ethan Dicks wrote:
> > On Thu, Jun 18, 2020 at 2:42 PM Peter Coghlan via cctalk
> > <cctalk at> wrote:
> > > Thanks for your reply Paul.  My eventual goal is to be able to use
> > > the synchronous serial interface on a MicroVAX to connect to IBM
> > > machines that only support bisync lines.

I trimmed this because we seem to be missing the crux of clocks and why we need them! 
In async data we take our clock from the edge of the start bit, and we have a stop bit which is really there to stop characters running together.
With sync we just have a stream of digital data and we use the clock to know when to sample it.
There are no stop bits or start bits, and we use a synchronisation character at the start of each block to work out how the characters are positioned in the data stream.
So we need clocks...

Its been ages since I did this but looking here

I see we have a transmit clock output on pin 24,  transmit clock input on 15 and RX clock input on 17.
So if on checking with a scope I have clocks on 24, I would try linking 24 and 15 on one side to 17 on the other side.
If you have only one clock running then that goes to 15 and 17 on both ends....
.... as suggested here:-

This assumes you have RS232 interfaces. If its X21 then eliminators are available on e-bay for a few pounds.


More information about the cctalk mailing list