sync serial cards

Ethan Dicks ethan.dicks at
Fri Jun 9 00:18:50 CDT 2006

On 6/9/06, Brad Parker <brad at> wrote:
> I think you could use a 56k CSU/DSU and something as simple a ISA bus
> card with an SCC on it.  There used to be a linux driver for the 8530
> that could keep up at 56k.  There are other higher performance cards of
> course.

The "problem" is that semi-modern machines have no ISA slots - my
primary knock-around-and-game machine at home has a motherboard from
early 2000, and it's 5 PCI and 0 ISA.

> The CSU/DSU might have a giant v.35 connector or if you are lucky just a
> DB-25.  I think some even had both (it's all dim now).

There are also DB-25 (EIA) to V.35 converters out there - we used to
sell a few by Gandalf when we'd sell a COMBOARD to V.35-equipped
customers (our cable was similar to a DEC serial cable - 40-pin BERG
on one end, DB-25 on the other, with 1488s and 1489s for EIA level

> You'll laugh, but I ran a 56k internet connection for a few years out of
> my house using a sparc station 1 running sun os.  I ran the sparc serial
> port (8530 based) directly into a CSU/DSU.

Pretty wild for the home.  The Qbus and VAXBI COMBOARDs used the Z8530
(the COMBOARD-I and COMBOARD-II for Unibus used a COM5025 since that's
what DEC boards in the late-1970s used for sync serial (DUP-11?)).  We
had either an 8Mhz 68000 or a 10MHz 68010, running a monolithic app
(one in C, several simpler protocols in 68K  assembler) driving one
Z8530 serial channel at up to 64Kbps all day long (with a pair of
boards in the back room we'd hooked up to a tweaked modem eliminator
at 128Kbps).  If your modernish PC can't keep up with a 56Kbps sync
data stream, your operating system is probably stepping on your
interrupts  - it's not a CPU-load thing.

I am unaware of a PCI-based sync serial card.  I think by the time
that PCI was becoming dominant, most folks who wanted a 56K line to
the outer world wanted it to speak ATM or ppp (not Bisync or SNA),
thus it was reasonable to offload that task to a TCP/IP router.  There
are *plenty* of models of Cisco router that you can hang a CSU/DSU off
of, but I don't recall ever seeing that done with a PCI-based machine.


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