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Technical Details.
ii.)
IRQ jumper block set in accordance with the special
IRQ handler software routines.
iii.)
CRA register setup appropriately.
iv.)
The correct interrupt vector in the PC's memory is
'stolen' and redirected to a handler.
All this is pretty esoteric and only of interest to the odd user.
__________________ _
REN Remote ENable.
To program REN for receive the data direction bit in
DDRB must be set LOW. REN is bit 1 of PORT B.
To program REN for transmit the data direction bit in DDRB
must be set HIGH. Setting bit 1 of PORT B HIGH sets REN
HIGH. Setting bit1 of PORT B LOW sets REN LOW. The
SLAVE command turns the REN line around from the normal
transmit output mode to receive input use.
SRQ Service ReQuest.
___________________ _
The SRQ line is bit 7 of PORT B. This is normally set for
input and the current state of the SRQ line can be read directly
from here. There is an SRQ input only line connected to CB1 of
the PIA. This line can only detect strobe pulses on the SRQ line,
NOT its current.
The SN75161 only allows SRQ receive when the computer
is acting as the system controller, ie ALL THE TIME in normal
use. To send SRQ use the SLAVE command to turn around the
SRQ direction and then use the SRQ, NO SRQ commands or
write directly to PRB.
An IRQ to interrupt the processor can be generated by
SRQ transitions by appropriate setting of CRB. Otherwise SRQ's
can be detected by polling the IRQB1 flag in bit 7 of CRB of
the PIA. Thus the PC IEEE can send a service request to an
external device and can easily detect them.
Chapter 3
PC IEEE Reference
Page 31
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