Appendix 1
*
SIGNAL
GROUND
*
ATN
GND
*
SRQ
GND
*
IFC
GND
*
NDAC
GND
*
NRFD
GND
*
DAV
GND
REN
DIO
8
DIO
7
DIO
6
DIO
5
*
TWISTED
PAIR
WITH
SIGNAL
CABLE
OF
SAME
NAME.
Fig. A 1.3 IEEE-488 Connector
Because the bus
is
asynchronous and a 'wired
Or'
open -
collector configuration is used for each line in the interface,
see Fig.
Al.2,
a handshake line can only change state after
all listeners have set their respective drives to the same
state. The data transfer rate can only be as fast as that
of
the
slowest receiver.
SERVICE
REQUEST
(SRO)
This
is
a dedicated bus line used by devices to indicate that
they need service from the controller (i.e. the opportunity
to 'talk').
To
establish which device has flagged the SRO, the control-
ler
will
use a mechanism called serial poll.
SERIAL
POLL
This
is
a special mode set by the controller
in
addressed
lis-
teners causing them, when permitted to talk, to put on the
bus their serial poll status word. Bit 6 of this word indicates
which device originated the SRO seen
by
the controller.
70
0
24
12
SHIELD
23
11
ATN
22
10
SRQ
21
9
IFC
20
8
NDAC
19
7
NRFD
18
6
DAV
17
5
EOI
16
4
DI04
15
3
DI03
14
2
DI02
13
1
0101
0
A typical host computer routine for the serial poll would
execute the following sequence of events, having sensed
the~
service request (SRO):
1. Set up a timeout interrupt, so as to deal with those
cases of polling a device which
is
not present;
2.
Enter a 'do-loop' of device addresses, and within this:
a) Send the commands
UNLISTEN, UNTALK,
MYLISTENADDRESS - this 'silences' the bus
and sets the controller to listen;
b) Command the addressed device
to
TALK;
c) Command SERIAL POLL ENABLE (SPE) -
this
will
cause the addressed talker to respond on
to the bus with its serial poll status word. Bit 6 of
this word indicates whether
or
not this device
originated the service request.
d) Having read the response, command SERIAL
POLL DISABLE, UNLISTEN, UNT ALK again
and proceed around the loop until the SRO
originator
is
found.
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