6-1
Circuit Descriptions
Section 6.0
6.1 Introduction
The information on the System 7500™ circuits
identifies specific signals and I/O ports that help
identify the appropriate signal levels. The text
uses the word SET for high and
CLEAR for
low. To locate the assemblies within the System
7500™, refer to Figure C-1. Use the system inter
-
connect (Figure C-2) to identify wiring harness
terminations and individual signals within a har
-
ness.
6.2 Display Panel Assembly [A5]
6.2.1 General Information
The System 7500™ has two 80C550 microcon
-
trollers with one dedicated to system control and
the other dedicated to system monitoring. The
control microcontroller (U15) sets the system
enables and control limits, while the monitor
microcontroller (U22) monitors system perfor
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mance and sets “inhibits” when an error is detect
-
ed. Both of these devices have “on-board” A/D
converters, and each device independently moni
-
tors separate analog signals. The address, data
and control buses of both microcontrollers are
isolated by a device called the “mailbox”, U19, a
dual port RAM that allows two-way data transfer
between the two microcontrollers.
On the schematic, the signal labels will have either
a “C” or “M” attached. The “C” is for the con
-
troller logic (U15) or control microcontroller and
the “M” is for the monitor logic (U22) or moni
-
tor microcontroller interface. The schematic for
the display panel (Figure C-8) is on four separate
sheets. Sheet 1 is the schematic for the displays
and drivers only; sheet 2 shows all the connec
-
tors that are on this assembly along with some
discrete non-logic circuitry; sheet 3 is the control
microcontroller logic; and sheet 4 is the monitor
microcontroller logic.
This assembly has several programmable logic
devices to interface signals to the data bus for
both the control and monitor microcontrollers.
During the discussion when two reference desig
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nators are listed together it means the logic func
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tions are the same and can be interchanged.
6.2.2 Mailbox [U19]
A dual port logic device that allows data transfer
between the two microcontrollers. All system
setups and messages are communicated through
the mailbox between the two microcontrollers.
When data is loaded by either microcontroller, a
bit labeled DA (data available) is
SET to inform
the other microcontroller that it has mail. Once
the data is read from the mailbox, another bit
labeled DC (data cleared) is
CLEARED to inform
the sender that the mail has been retrieved. Each
microcontroller has independent access to the
mailbox and the mailbox is the only component
that connects the two data buses. Each instruc
-
tion sent through the mailbox requires two bytes
of information with the first byte (command byte)
identifying the instruction and the second byte
(data byte) containing the data for the instruction.
6.2.3 Power Adjustments [U16 & U33]
(C for controller; M for monitor) Power adjust
-
ments on the System 7500™ are made by rotat
-
ing the power control encoders. The mnemon
-
ics for the power encoders are: CT - CUT; CG
- COAG; BP - BIPOLAR; BM - ABC™. Each
encoder is a two bit counter (i.e.; CT0 & CT1)
where the two counts are used to identify the
direction of rotation. The logic devices (U16
& U33) store the previous count and compare
it to the new count in order to recognize if the
count is increasing or decreasing, which defines
if the encoder is rotated clockwise or counter-
clockwise. Each “click” of the encoder is a count
and the number of counts are stored within
U16 & U33 until the microcontroller reads the
port. The microcontroller reads each encoder
port independently and if an encoder has been
rotated, the count will be greater than zero. The
data the micro will see is a number (0 to 32) rep
-
resenting the number of “clicks” the encoder has
been rotated and a separate bit that signifies the
direction of the count, either up or down. The
microcontroller then takes this count and adds or
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