NorthStar HORIZON Manual page 69

various interrupt requests
VI0/-VI7/, PINT/ and NMI/.
request an interrupt.
to the backplane interrupt lines
These lines are pulled low to
7. Baud Rate Generation and Selection: A total of 9 baud rate
clocks from 9600 x 16 to 75 x 16 are generated on the
motherboard from the S-100 bus 2 MHz clock. These are
divided down to the proper frequencies by four 4-Bit
counters, types 74LS161 and 74LS393, at locations 5D, 6D,
and 7D. The baud rates generated are available at the Baud
Rate Selection header at 2D. The clocks are expressed
sixteen times the desired baud rates since this is what the
USARTs generally require.
8.
Real-Time Clock: The real-time cloc\ feature allows
generation of a signal to the processor at regular
intervals. The clock intervals are divided down from a tap
on the baud-rate generator by the MOS 14-bit counter at 10B,
a type 4020 IC. A total of 12 intervals from 3.328 ms. to
27.263 seconds are available at the Clock Rate header at
lOA. The selected clock is used to set the clock flip-flop
(CLK-FLG). The clock flag
conditio~
is available to
programs as a bit in the motherboard status byte. The clock
flag condition can also be jumpered to request an interrupt.
69 North Star HORIZON Computer