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98640A Analog Input Interface
The A -to-O card receives the register address on address lines A 1 through A6 of the backplane. It
decodes these lines as follows:
A
1
through A
3
specify which channel to read.
A4 and AS specify what gain to use for the reading:
A5
A4
o
0
o
1
1
0
1
1
Gain
1
8
64
512
A6 indicates whether it is an analog read. If A6 is set to 1, it is an analog read; if it is set to 0, it is
an ordinary access to the ID, status, or pace timing register.
AO is ignored. Odd-numbered register addresses are used for accessing low bytes; since only full
words are read by analog reads, odd-numbered addresses are not used. (There is no AO line on the
backplane; its function is handled by BUDS- and BLDS- which are, as the minus signs indicate,
negative true.)
For example, if your read request specified a register address of 86, the A -to-D card would break
down that address as follows:
A6
1
analog
read
A5
A4
o
1
gain
=
8
A3
A2
A1
0 1 1
channel
=
3
AO
o
not
used
The value returned by an analog read is the voltage for the channel and gain specified tWQ..ana]og
r£.adurevicm.~.
Thus, you must take n+2 readings to get the n values that you desire. For instance,
the example program below takes 3 readings to get 1 valid voltage value. (Refer to the paragraphs on
the "Analog Pipeline" at the end of Section 3 for a fuller explanation of this phenomenon.) The for-
mat for the returned value is:
15 14 13 12
11 10
9
8
7
6
5
4
3
2
1
o
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
I BIWIOI SIDIDIDIDIDlolDIDIDIDIDIDI
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
where.:
B
=
busy
W =
wait
MSB
LSB
o
=
common mode overrange
(0
=
overrange;
1 =
no overrange)
S
=
sign
(0
=
positive;
1
=
negative)
D
=
data
(Refer to Section 3 for a more detailed explanation of the data word.)
4-2
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