EPC-23 Hardware Reference
IRQ3 through IRQ7, IRQ9, IRQ11, IRQ12, IRQ14, & IRQ15 (I)
Interrupt requests 3 through 7, 9, 11, 12, 14, and 15 are used to signal the
microprocessor that an I/O device needs attention. The interrupt requests are
prioritized, with IRQ9, IRQ11, IRQ12, IRQ14 and IRQ15 having the highest priority
(IRQ9 is the highest), and IRQ3 through IRQ7 having the lowest priority (IRQ7 is the
lowest). An interrupt request is generated when an IRQ line is raised from low to
high. The line is high until the microprocessor acknowledges the interrupt request
(Interrupt service routine).
LA17 through LA23 (I/O)
4
4
These signals (unlatched) are used to address memory and I/O devices within the
system. They give the system up to 16M of addressability. These signals are valid
when BALE is high. LA17 through LA23 are not latched during microprocessor
cycles and therefore do not stay valid for the whole cycle. Their purpose is to
generate memory decodes for 16-bit, 1 wait-state, memory cycles. These decodes
should be latched by I/O adapters on the falling edge of BALE.
-MASTER (I)
This signal is used with a DRQ line to gain control of the system. A processor or
DMA controller on the I/O channel may issue a DRQ to a DMA channel in cascade
mode and receive a -DACK. Upon receiving the -DACK, a microprocessor may pull
-MASTER active (low), which will allow it to control the system address, data, and
control lines (a condition known as tri-state). After -MASTER is low, the
microprocessor must wait one clock cycle before driving the address and data lines,
and two clock cycles before issuing a Read or Write command. If this signal is held
low for more than 15 microseconds, the system memory may be lost because of a lack
of refresh.
-MEMCS16 (I)
The '-memory 16-bit chip select' signal indicates to the system that the present data
transfer is a 16-bit memory cycle. It must be derived from the decode of LA17
through LA23. -MEMCS16 is active low and should be driven with an open
collector or tri-state driver capable of sinking 20 mA.
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