DESCRIPTION AND OPERATION
DIGITAL I/O
I-E96-316A
2 - 3
The A/D converter block circuits change the input signal to a
12-bit value that is sent to the slave expander bus interface.
This value is an analog count that corresponds to the input
voltage. Nominal input range is 1 to 5 VDC, however, it allows
for a 0.75 to 5.25 VDC input range which is
±
6.25 percent of
the nominal input range span (4 VDC).
Analog Input Circuit Calibration
The reference block generates accurate 1 VDC and 5 VDC sig-
nals. The QRS does not have potentiometers to adjust zero off-
set and gain for the A/D converter circuits. Instead, the MFP
reads the reference voltages once per minute to calibrate the 0
percent (1 VDC) and 100 percent (5 VDC) points; this calibra-
tion automatically corrects the measured values. It is per-
formed continuously to correct for drift and temperature
variations.
Analog Outputs
The D/A converter block is two separate D/A converters. Each
D/A converts a 10-bit digital value (analog count) from the
MFP to an analog output (1 to 5 VDC). To check module circuit
integrity, the outputs are fed back to the analog input section.
The feedback values (analog output digital values) are com-
pared to the values that were sent to the analog output section
to test the output quality. This tests for an output circuit fail-
ure or an open loop between the master module and slave
module.
The analog output default switch (S2) sets the output values
during system start-up or
time-out
(refer to
in this section). The analog outputs will go to 0 percent or 100
percent output, or they will hold their current values depend-
ing on the setting of S2.
explains how to set S2.
The analog output mode switch (S3) sets the type of output,
either current or voltage. If current mode is selected, the E/I
circuits on the QRS convert the voltage from the D/A converter
to a current output.
explains how to set S3.
DIGITAL I/O
The QRS can input three separate digital signals and output
four separate digital signals. Digital inputs are voltages of 24
VDC, 125 VDC or 120 VAC. These voltages indicate an ener-
gized (ON) field device; a 0 volt input indicates a de-energized
(OFF) field device. The QRS digital outputs can switch 24 VDC
at 250mA. Figure
shows the digital input and output cir-
cuits.
