Re: Using modem on IIGS

[dempson@actrix.gen.nz (David Empson)]

Supertimer <supertimer@aol.com> wrote:

> "Jack Somers" <jsomers@unlinfo.unl.edu> wrote:
> 
> >What kind of card can I put in my Apple IIGS to use any external modem with
> >it?  And will there be a speed limit?  In other words, will I be able to use
> >a 56k modem with it?
> 
> Contrary to what some have been saying, you can use a 56k modem
> at 56k on an UNACCELERATED IIGS using just a Macintosh
> hardware handshaking modem cable.  You'll need ProTERM A2
> from http://www.intrec.com/ Intrec to actually run the port and the
> modem from this speed.

Note that you won't be able to get anything like the full throughput of
a 56K modem on a IIgs, since you would typically be using error
correction and data compression, resulting in typical comms speeds of up
to 6500 bytes per second on a computer with faster serial ports
(requiring a modem-to-computer link of 115200 bps or faster to keep up).

> 56k is NOTHING if you think about it.  The IIGS can do Appletalk
> through the serial ports.  That's 230k, some 4x faster.

Well, yes, but it doesn't work in anything resembling the same way.
AppleTalk gets an interrupt when a packet starts coming in, then hogs
the CPU until the entire packet has arrived (up to 20 milliseconds
later).  It also doesn't have to deal with simultaneous transmit and
receive.

Standard serial software gets interrupted for each character as it
arrives.  With the port running at 57600 bps, full speed incoming data
arrives every 173 microseconds.  On an unaccelerated IIgs, the driver
would have to be written very carefully to cope with this, and it won't
leave much CPU time for doing anything significant with the data.

One trick the driver could pull would be to lock out interrupts for a
short time while doing data processing, e.g. up to 500 microseconds at a
time.  It will then get interrupted with anything up to three characters
pending, which can be processed in a single interrupt (less overhead).
This is due to the three character receive buffer in the SCC.

Another possible solution is to poll data out of the SCC in the
background, as long as it can be handled fast enough (e.g. while data is
being streamed in for a file transfer protocol).

In any case, it is possible, but a lot easier on an accelerated machine
with more CPU time to spare.

> Then there is the IIGS disk drives, that's about 300k (30KB/sec).

Actually, the raw data rate of the 5.25" drive is 250 kilobits per
second (4 microseconds per bit cell), or 500 kilobits per second for the
Apple 3.5 drive.  There is some data encoding/decoding required, which
slows down the usable transfer rate somewhat.

> Then there are hard drives which can theoretically pump 1MB (Apple Bus
> limit) using a DMA SCSI card.

Hardware does that, not the CPU.

> The only time 56k (or even 33k) will cause you problems is when
> you have your IIGS heavily loaded with interrupt eating desk
> accessories, inits, etc.  These things may also screw up Appletalk.

Definitely.  I can't get anything faster than 19k2 working reliably on
my 8 MHz ZIPped IIgs if I have Appletalk turned on, though that is in
Spectrum, which is likely to have more overheads than ProTerm due to
running under GS/OS.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand