User's Manual

CHAPTER 3: Software Configuration

Macintosh

computers do not use UARTs. The Macintosh SE through

IIfx models use a Zilog

Z8530 chip called a Serial Communications

Controller, or SCC, that has a maximum speed of 57,600 bps. This

speed can be compromised by other serial communications, including

printer transmissions and Appletalk

, the networking software that

allows Macintoshes to share files. When Appletalk is active it controls all

serial communications on the Macintosh. Because it gives priority to

network communications, it may lose modem data at higher

transmission speeds. Therefore, when Appletalk is active you risk losing

data on serial port communications over 2400 bps, though most users

can work up to 9600 bps without problems. Other activities that could

cause the serial driver to drop bits at high speeds include the floppy disk

driver formatting a disk, the CPU paging in or out in virtual memory

mode, and the Mac

IIci or IIsi running the on-board video in 8-bit

mode. Therefore, for maximum communications speed on the

Macintosh, we recommend as few concurrent activities as possible. To

use V.42bis compression at 19,200 bps or faster, we recommend that you

install a high speed serial port card. Newer Macintosh computers, such

as the Quadra and Centris models, support serial port speeds up to

115,200 bps.

3.2.1 How Can You Identify Your UART Type?

If you have MS-DOS 6.0 or later, you can find your UART type from

a diagnostic program called MSD. To use it, type MSD at the DOS

prompt. After the opening screen, select COM Ports.... The last line of

the report tells you what type UART you have for each COM port. MSD

does not distinguish between the 8250 and the 8250A. However, if you

have an IBM AT or newer computer, you likely have an 8250A or 16450

UART installed, both of which are reliable to 19,200 bps. If you would

like more detailed information about your UART than MSD can

provide, you can download shareware UART identification programs

from the World Wide Web, from the IBM Communications Forum on

CompuServe

, or from a local BBS.