3. STD Bus Interface
25
Maskable Interrupts
The STD bus maskable interrupts monitored by the ZT 8904 are INTRQ* (P44),
INTRQ1* (P37), INTRQ2* (P50), INTRQ3* (E67), and INTRQ4* (P5). These maskable
interrupts are routed to a jumper configuration block (W17-22) for added flexibility. Note
that an STD 32 backplane is needed to use INTRQ3*.
The ZT 8904 is also capable of generating STD bus interrupts. This feature is useful in
multiple master systems to coordinate communications between processors.
Some applications may find it necessary to share multiple interrupt sources on a single
STD bus interrupt request, as shown in the "
STD Bus Polled Interrupt Structure
" figure
following. Since the interrupt controller provides a single vector for each input, it is up to
the application software to poll each possible source on the shared interrupt request
signal to determine which is requesting service. This procedure is fine for most
applications, provided that each source can be polled and that the interrupt controller is
programmed for level-triggered operation.
ZT 8904
STD BUS
INTRQ*
INTRQ*
INTRQ*
INTRQ*
INTERRUPT STATUS
PORT
INTERRUPT
SOURCE N
I
S
P
INTERRUPT
SOURCE 2
I
S
P
INTERRUPT
SOURCE 1
I
S
P
STD Bus Polled Interrupt Structure
Some applications include edge-triggered interrupt sources. For example, the Ziatech
Industrial BIOS uses edge-triggered interrupts to support the timer used to generate the
periodic system tick. Since the interrupt controller inputs are not independently
programmable for edge-triggered or level-triggered interrupts, all inputs for these
applications must be treated as edge-triggered.