Vector Graphic Bitstreamer II, User Manual, Page: 40 / 54

an interrupt, in combination wi th one or more of the 8251 interrupt sources,
you have to "open collectorize" the real time clock line.
In fact, you must
open collectorize the real time clock line if ANYboard in the computer is
wi red to generate interrupts, and you \vant to use the real time clock for
interrupts. In Vector Graphic computers, the Flashwr iter boa rd is often
wired to generate interrupts.) To open collectorize the real'time clock
line, you .vill use one of the two spare open collector N~~D gates, which are
accessed
in jumper area B. Tie a j~~per from pad A-I to pad 8-1, install a
small jLm1per from pad B-1 to B-2, and install a third jumper from B-3 to
A-8. (This also inverts the clock signal, but this has no effect on the
syste-a.)
If you \vant to be able to software mask (enable and disable) the real
time clock interrupt, then you have to do the following jumpering, even if
the real tiJ(\e clock is the only interrupt: Tie a jumper from ?in 10 of chio
U-9 to pad 8-2. Then, install jumpers from pad A-I to B-1, and from B-3
to
A-8, as in the ?recedin3paragraph, but do not put a j~~per becween 8-1 and
B-2.
Important: If there is more than one Bitstreamer II board in the system,
only one of them should generate real tirneclock interrupts.
The board requires that you software rnask (enable and disable) the TxRDY
and RxRDY interrupt sources that have been connected in area A. If you
wired the real time clock interrupt to be software lnaskable, then naturally
it too must be software masked. To mask the interrupts, you output a byte
to Parallel Channel A, which is I/O address x8 (usually 08). This is the
"inter rupt masking re9iste r. " The cho ice of enabled and disabled lines
remains the same until the next time you send a byte to the interrupt
masking register. Each bit corresponds to one interrupt source, as listed
in Table 8. A 1 enables the corresponding interrupt source, and a 0
disables the source. It does not matter what you put in bit position 7,
'Nhich is not used. For example, assume you have wired up all the interrupt
sources. To enable only serial channel B ~hRDY, serial channel C RxRDY, and
the real time clock interrupt sources, output 64H to I/O address x8. 64 in
hexadecimal notation equals 01100100 in binary, which has l's in bit
positions 2, 5, and 6 only (reading fran the right, beginning with position
0).
Outputting 7F to I/O address x8 enables all t:'1e interrupt sources.
Remember: if you are using any software maskable interrupts, the output
side of Parallel Channel A cannot be used for any other purpose, because
each time you send a byte to it, you will change which interrupts are
enabled or disabled. You MUST initialize ~~e interrupt masking re3ister at
the start of any progra~ usin3 b~ese interrupts.
IE you are using an 8080 CPU, the interrupt source is required to put a
byte of data on the S-100 input data bus at about the same time the
interrupt is sent. Specifically, it has to be put on the bus by the time
the CPU activates t..;"'e
sn,IT.~
li;1e
L:1
resp)!1seto b'leinterrupt. This byte of
data has to be a single executable instruction, because the 8080 will
execute it. Normally, it is one of the 8080 RST (restart) instructions,
which cause the processor to call a program at a ·specific address determined