The Wizard of Oz <wizard@emeraldcity.gov> wrote:
Wendell III wrote:
Got it -- so would a physical Localtalk <-> Ethernet bridge not work in
place of Linux PPC with a Localtalk port? I don't have an actual old Mac...
I was looking at it a couple of years ago on a Linux PC (not PPC).
Before something drew my attention away I was thinking it might have
been possible to use a standard serial port on the PC since I had
connected a GS to it years earlier as a terminal. Maybe someone can
comment on this.
You certainly can't do LocalTalk through a standard PC serial port.
LocalTalk operates using a bit-synchronous transmission protocol based
on SDLC framing, running at 230400 bps, with FM modulation (I forget
whether it is FM0 or FM1 without looking it up). It also depends on some
specific features of the Z8530 SCC (Serial Communication Controller) or
derivatives, some of which might be implemented by other chips that
support the same physical protocol.
Standard PC serial ports only operate in asynchronous mode, and have a
maximum speed of 115200 bps, so you can't even match LocalTalk's speed.
Once you allow for asychronous vs HDLC framing, LocalTalk's peak
throughput (for maximum length frames with no bit-stuffing) is in the
order of 28500 bytes per second, whereas a PC serial port's peak
throughput is only 11520 bytes per second.
With a PC serial port which supports higher baud rates but is otherwise
standard, you could theoretically do LocalTalk with some external
hardware to do a protocol translation between SDLC and asynchronous, as
long as the serial port could run faster than 230400 bps.
A nonstandard PC serial port which used the Z8530 (or a later
derivative) could do LocalTalk directly, but it wouldn't work with
software which depends on the low-level characteristics of the standard
PC serial chips.