
2-26
Functional Description
bits in the SSTAT0 tell which SCSI interrupts occurred and determine
what action is required to service the interrupts.
3.
If only the DIP bit is set, read
DMA Status (DSTAT)
to clear the
interrupt condition and get the DMA interrupt status. The bits in
DSTAT tell which DMA interrupts occurred and determine what action
is required to service the interrupts.
4.
If both the SIP and DIP bits are set, read
SCSI Status Zero (SSTAT0)
and
DMA Status (DSTAT)
to clear the SCSI and DMA interrupt
condition and get the interrupt status. If using 8-bit reads of SSTAT0
and DSTAT registers to clear interrupts, insert a 12 clock delay
between the consecutive reads to ensure that the interrupts clear
properly. Both the SCSI and DMA interrupt conditions should be
handled before leaving the ISR. It is recommended that the DMA
interrupt be serviced before the SCSI interrupt, because a serious
DMA interrupt condition could influence how the SCSI interrupt is
acted upon.
5.
When using polled interrupts go back to Step 1 before leaving the
ISR in case any stacked interrupts moved in when the first interrupt
was cleared. When using hardware interrupts, the IRQ/ pin is
asserted again if there are any stacked interrupts. This should cause
the system to re-enter the ISR.
2.7 SCSI Bus Interface
The SYM53C710 can be used in both SE and differential applications.
In SE mode, all SCSI signals are active low. The SYM53C710 contains
open drain output drivers that can be connected directly to the SCSI bus.
Each output is isolated from the power supply to ensure that a powered
down SYM53C710 has no effect on an active SCSI bus (CMOS “voltage
feed-through” phenomenon). Additionally, signal filtering is present at the
inputs of REQ/ and ACK/ to increase immunity to signal reflections.
In Differential Mode, the SDIR [7:0], SDIRP, IGS, TGS, RSTDIR,
BSYDIR, and SELDIR signals control the direction of external differential
pair transceivers. See
Figure 2.9
for the suggested differential wiring
diagram. Please refer to
Appendix D
for more information. The wiring
diagram shows five 75ALS170 3-channel transceivers and one
75ALS171 3-channel transceiver, though other single and multichannel