Re: Network for the GS?

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

In article <4mefjt$r12@samba.rahul.net>,
Matthew Carlson  <carlson@rahul.net> wrote:
> David Empson (dempson@atlantis.actrix.gen.nz) wrote:
> :
> : [LocalTalk is] 230.4k synchronous, so only 8 bits are sent for
> : each character and the theoretical maximum transfer rate is 28800
> : bytes per second (the equivalent of an asynchronous serial link at
> : 288k bps).  The bandwidth is limited by packet and protocol overheads,
> : other network traffic, reaction time of the file server, reaction time
> : of the workstation, etc.
> 
> Wait a minute here: 230.4k synchronous?!!!!!!
> 
> Appletalk on a Mac may be that but, on a IIGS non-accelerated?

LocalTalk is the same on all machines - how else could a IIgs and Mac
talk to each other?

> Let us think about this, a 2.8mhz machine (2.5 or so with the ram refreshing)
> that is able to transfer 1 byte in two cycles.?? 

230400 BITS per second, not BYTES per second.  A maximum of 28800 BYTES
per second, as I said in the earlier article.

LocalTalk is packet-based.  The receiving computer is interrupted for
the first byte, and has to be able to respond within 100 microseconds,
or it will lose the byte (the Z8530 has a three-byte receive buffer).
The interrupt handler then remains active while receiving the whole
packet, which can take up to 25 milliseconds (something like 580
bytes).

This is obviously going to cause problems with serial comms on the
other port - 25 milliseconds is enough time for 50 bytes to be
received at 19200 bps.  As long as your comms program uses the
standard IIgs serial firmware, it shouldn't lose anything, because the
AppleTalk firmware polls the other port while it is receiving a packet.

If the comms program has its own interrupt handler, then data
received at the same time as an AppleTalk packet will be lost.

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