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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.