Vector Graphic Bitstreamer II, Руководство пользователя

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2.2 Asynchronous serial baud rate selection
Independently for each serial channel, you select ~~e desired baud rate
through a combination of hardware
s•.••
itches and software. If you are using
the standard operating systems and Extended Systems Monitors for Vector
Graphic Systems, however, you need only be concerned with the hardware
switches.
'!he hardware switch for each serial channel is located in the upper left
hand corner of the board. They are labeled as a group "Baud Rate Select"
and individually as "Chan-c," "Chan-B," and "Chan-A," from left to right.
For each serial channel, the switch contains eight rockers labeled "1" to
"8" and also labeled "9600," "4800," "2400," "1200," "600," "300," "150,"
and "110." The switches are in addition labeled "OPEN" on the left side.
To select one of the labeled baud rates for a particular serial channel,
press the desired rocker down on the right side, i.e. on the side opposite
the "OPEN" designation. Then press all other rockers within the same switch
down on the left side, Le. toward the "OPEN" designation. The resul t must
be that one rocker is down toward the right, and all others are down toward
the left. Otherwise the serial channel will not work at all.
If you are not using a particular serial channel, do not worry about the
position of its baud rate switch.
The labeled baud rates assume that the corresponding 8251 will be
initialized for a clock factor of 16. (It creates the baud rate by dividing
the clock input
by
16.) This initialization is handled in software, as
described in the 8251 references given in Perspective. All standard Vector
Graphic software, unless otherwise do~mented, use a cloCk factor of 16, and
therefore the labeled baud rates are correct.
However, custom software can use a cloCk factor of 64. If this is the
case, the actual baud rate will be 1/4 of the baud rate selected on tl1e
switch. (The only two asynchronous clock factors allowed by ~~e 8251's are
16 and 64.) Further, it may be desired to create software which allows the
operator to choose between two baud rates that differ by a factor of 4. For
example, the software may be designed in conjunction wi~~ a modem that can
accept signals at either 300 or 1200 baud. In this case, you would create
the software so that the operator's input determines whether the 8251 is
initialized with a clock rate factor of 64 or 16, respectively.
Remember, if you are using standard Vector Graphic software, do not worry
about this software option. You simply set the baud rate for a desired
serial channel
by
setting b~e baud rate select switch on the Bitsreamer II
board.