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Chapter 3 - Configuration
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 IBM Communications
Forum.