Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!jumbo!murray
From: murray@jumbo.dec.com (Hal Murray)
Newsgroups: comp.sys.mac.programmer
Subject: How fast can a MAC-SE drive a serial line?
Keywords: serial, fast
Message-ID: <13655@jumbo.dec.com>
Date: 28 Mar 89 03:16:51 GMT
Organization: DEC Systems Research Center, Palo Alto
Lines: 36

AppleTalk runs at 256K bits/second. I was expecting the vanilla serial
driver to be able to go as fast.

I connected the send side of the modem port back to the receiver and wrote
a simple test program that uses ASYNC writes and simple/sync reads. It
starts a write. Then, while polling for the write to finish, it reads
whatever is available. At 19.2K it works as expected: I get almost the
full speed when sending to myself. I do go around the poll/read loop
several times for each send and I receive all the characters I send. The
send side is only single buffered, but each buffer is 1000 characters so
the gaps before starting another send don't mess up the timings much.

When I turn the clock speed up to 56K, things no longer do what I want. It
acts like the interrupts are using the whole CPU: mouse tracking goes
crazy and the send is already finished when it returns. I haven't hooked
up a scope to see how fast the characters are actually going out, but my
program prints out 30K bits/second. When I unplug the echo jumper, I do go
around the polling loop and the mouse is happy. (But of course, I don't
receive anything.)

Am I missing something dumb? How does AppleTalk work? Does an SE get real
sluggish when sending or receiving an AppleTalk packet?

What I want to do is grab several (say 6 to 10) seconds of data as fast as
I can without missing any characters. They will all fit into memory. I'm
willing to disable all interrupts if necessary. I can process the info
after collection.

Is there some easy way to do this, and/or how fast can I go with the
normal driver? I've worked with the SCC chip before, so writing a
non-interrupt polling loop shouldn't be too hard.

The data I want to grab is coming from an A/D. I'm building the hardware
myself. It only takes 3 or 4 chips to serialize and level convert so that
seemed like a reasonable way to go. Would the SCSI port be simpler at
these speeds? I've never worked with either the SCSI hardware or software.