DDCMP sync?

Bob Smith bobsmithofd at gmail.com
Mon Jan 25 20:26:28 CST 2021

Usually, we referred to the synchronous chip as a USynrt chip, a play on UART,
DMC can run 1.544 Mb/s, or full T1. DUP or DU11 were good to upto 56K.
Bisync, ADCCP, HDLC, DDCMP, BDLC were all options back in 77 but...
Yes, they are slow.  I had a hard time trying to explain th the
Signetics design engineer who was tasked to copy my line unit design
into silicone that it had to run at least T1. New grad, no practical
experience yet, finally had to pull - I am the customer, I am paying
for this, if you cant do this, I will have you replaced.
He stuck with the project and at the end, said, ah now I understand.
just some background. USB is much more useful now a days.

On Mon, Jan 25, 2021 at 8:46 PM Paul Koning via cctalk
<cctalk at classiccmp.org> wrote:
> In playing with DECnet I built a DDCMP implementation which deals with a byte stream, normally from a UART.  So that works nicely with async link DDCMP as found in RSX and several other operating systems.  But the speed is limited.
> The other option would be synchronous links, which would enable connections to DMC11 or the like at speeds up to 1 Mb/s.  But synchronous comm devices that connect to modern computers aren't so easy to find, though I have seen a few.
> After playing with Arduino for LK201 keyboard emulation I started to wonder if one could be made to be a synchronous comm link with a USB back end, with low level things like byte framing and maybe DDCMP packet format handling in there, but the protocol state machine in the host behind the USB interface.  For moderate speeds that seems entirely practical.  For 1 Mb/s, probably not, though perhaps one of the fast ARM based units with its built-in SPI could be warped into that.
> The alternative would be something like a BeagleBone Black (or Green) such as David Gesswein used as the engine for his MFM hard disk emulator.  That clearly could do the job without any strain.
> So I'm wondering: would there be interest in such a thing?  If yes, should it be a modem-connected one (RS232 signaling, bit clock supplied externally by a modem or modem-eliminator)?  Or should it be the "integral modem" short distance type, the ones that used a pair of coax with 4-pin AMP connectors like this https://www.digikey.com/en/products/detail/te-connectivity-amp-connectors/206060-1/15588 ?
>         paul

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