DESCRIPTION AND OPERATION
SAC STATION I/O CIRCUITRY
2 - 14
I-E96-117B
®
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Bypass circuitry.
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Electric drive circuitry.
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Analog output circuitry.
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Analog input circuitry.
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Switching power supply.
The microprocessor provides the station with intelligence and
communication capability. It supports one serial port, a
four-channel A/D conversion, general I/O lines, and an
address/data bus that interfaces the microprocessor to mem-
ory and peripheral devices. There is also a built-in machine
fault timer (MFT) function. This halts the station if the software
fails to write to the timer every 1.5 seconds. The microproces-
sor provides a serial interface that enables it to send set point
and control output data, and receive update messages through
the RS-485 port to the MFP module.
Feedback from the analog input passes through a 60-hertz fil-
ter, differential amplifier and error compensation amplifier
before it reaches the microprocessor. The microprocessor has
an internal analog to digital converter (A/D conversion) that
converts the analog inputs to a digital word. The microproces-
sor may use this information to update displays or send to the
MFP module in a station update message.
The bypass circuit is a microprocessor controlled four to
20-milliamp analog output that provides analog output backup
for the CIS outputs. This circuitry also puts an electric drive in
manual operation if raise/lower (R/L) contact control is
desired. Electric drive circuitry contains two relays that pro-
vide contact outputs for the electric drive raise/lower signals.
Membrane pushbuttons control the electric drive relays
directly.
The display driver continuously updates the faceplate displays
with system process information from the MFP module
(through the serial interface), process variable, control output,
set point, operational mode and error status.
Bypass Circuitry
Bypass circuitry on the analog control station interfaces to the
CIS analog output or electric drive. The SAC station bypass cir-
cuitry provides five bypass functions. Refer to Table
for a
list of the five bypass functions and a brief description.
In normal mode of operation, the station analog output matches
the last received control output value from the MFP module. The
station analog output is a closed loop controlled current source
with the output shunted to ground by Q1 (see Figure
