Dave Huang <khym@bga.com> wrote:

>On Sun, 14 Mar 1999, Maurice Smiley wrote:
>> Umm...it works on the MacOS side.  In fact, 3Com lists the entry level Mac
>> for running this as a Mac Plus running System 7.  So, it would seem that the
>> issue is indeed NOT the hardware being able to drive 115200, but either a
>> NetBSD configuration issue, *OR* NetBSD not supporting serial port speeds
>> over 57600.
>
>Really, it's the hardware. While it's true that NetBSD/mac68k doesn't
>support serial port speeds over 57600, even on the (two) Macs that do
>support it, that's not the issue with your system... I'd guess that on
>the MacOS side, it's running the serial port at 57600 baud.

Quoting from the "Guide to the Macintosh Family Hardware", Second Edition,
 Chapter 10 Serial I/O Ports, page 363:

"The maximum nominal data transmission rate that you can select through 
the Macintosh Toolbox is 57,600 baud. This is the maximum rate that the 
classic Macintosh computers can maintain for tranmission of serial data 
when the SCC port is operating in an asynchronous, interrupt-driven 
fashion, timed by the 3.672 MHz clock.

AppleTalk operates at a nominal data transmission rate of 230.4 Kbaud. 
This higher rate is possible because AppleTalk communications are not 
interrupt driven; during AppleTalk communications, the AppleTalk Driver 
has complete control of the computer. Although AppleTalk uses a 
synchronous communications protocol, the AppleTalk Driver runs the SCC 
chip in asynchronous mode, timed by the 3.672 MHZ clock.

The maximum possible transmission rate for serial data ranges from 
approximately 500 Kbaud on the Macintosh Plus and Macintosh SE to 900 
Kbaud on the Macintosh II family. To achieve such data transmission 
rates, the SCC would have to be operated in synchronous mode timed by an 
external clock, and the serial driver would have to have complete, 
uninterrupted control of the computer."



This seems to say to me that the hardware is capable of asynchronous baud 
rates higher than 57.6K, but only when run with dedicated code that is 
not interrupt driven.  That's not too compatible with a multi-user system 
like NetBSD, but may be acceptable for a single-user system like MacOS.  
Rates above 230K are also possible in a synchronous mode, which I believe 
requires that both transmitter and receiver run on the same clock source 
rather than their own internal clocks like an asynchronous mode does.  
One could also divide the internal receive clock by something other than 
the default 16 and achieve a higher baud rate at the risk of missing 
incoming characters due to clock jitter, but the increased interrupt load 
would probably swamp most 68k Macs.

-bob