Vector 4 - Page 107

VECTOR GRAPHIC, INC,
B. Baud Rates *
*
During the initialization process all USARTs have their baud factors set. The
actual Baud Rate for each USART is determined by this factor coupled with an
externally generated timing signal.
The Keyboard USART uses a specially designed circuit (<D13>) to generate its
timing signal. This circuitry in combination with the USARTs pre-determined
baud factor allows the keyboard USART to operate at 300 baud.
The other two USARTSs receive their timing signal from ports 11H and 12H of
the Timer chip. During initialization these ports are configured so they
generate a timing signal which allows the modem port to operate at 300 baud
and the serial printer port to transmit/receive at 9600 baud. These rates can
be changed by reprogramming the timer -and/or changing the USART baud
factor. The following chart shows how the baud rate is changed by
reprogramming the timer.
1. Initialize the USART timer ports. This includes: Selecting mode 3
operation, reading/writing in high/low byte format, configuring counter to
binary counting.
T
1
(for modem) Send 7T6H to port 13.
T9
(for printer) Send B6H to port 13.
2. Set Delay Count for each USART port by sending out low order and high
order bytes. Refer to Baud Rate Chart (on next page) to obtain proper
delay count value. For this example each USART will be set to generate
110 baud.
Ty
(for modem) Send 70H to port 11H. Send 04H to port 11H.
Ty
(for printer) Send T0H to port 12H. Send 04H to port 12H.
See VECTOR 4 PROGRAMMERS GUIDE for more detailed information on how
baud rates are selected.
II 5-16 7200-0001 09-01-82