Xycom XVME-560, Manual

Page: 35 / 70

XVME-560 Manual
September, 1984
3.6 DATA
ACCESS LOCATIONS
The A/D converter produces a digital output which corresponds with the applied analog
input at the selected channel. The digital output is stored at locations base+86H (high
byte) and base+87H (low byte).
The digital information must be read in the order high byte first, low byte second or
both bytes can be read at once with a ‘16-bit
"word" read. This
is because in the
Sequential or Single Channel
conversion modes reading
the
lower eight bits of
data will
initiate a new A/D
conversion,
and thus may
write over
the present
high
byte.
3.6.1
Digital Data Format
The AIN digitizes the value of the analog signal on the
input
of the selected channel.
The analog equivalent of this digital number depends upon which digital data format
and input range have been previously jumper selected (see Section 2.4.9).
The digital data format is first determined by the choice of the input scaling range:
Unipolar (e.g., 0 to 5V) or Bipolar (e.g., -5 to +5V) (see Section 2.14).
If the input scaling range is unipolar, then a "straight" binary data format must be
selected. If the input scaling range is Bipolar, then the data format can be selected as
two’s complement or offset binary.
3.6.1.1
Unipolar Digital Data Format
An example of the unipolar digital data format (straight binary) with
the
input scaling
range set for 0 to 5 volts is shown in Table 3-5 below.
Table 3-5. Example of 0 to 5 Volt Input Range
Encoded in Straight Binary
I
High Byte Low
Byte
D15 DO
4095
Full scale
0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 5V*4096(-1LSB)
Zero
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 OV (+1/2 LSB)
.
NOTE
The value of one "LSB" is the input voltage change
that causes an increase or decrease of the digital
(binary) input value by one bit.
3.6.1.2
Bipolar Digital Data Format
Examples of the bipolar digital data formats (offset binary and two’s complement) with
the input scaling range set for -5 to +5 volts are shown in Table 3-6 below.