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FORM 160.54-M1
ISSUE DATE: 10/25/2019
SECTION 5 - MICROBOARD 031-03630-001
JCI COMPANY CONFIDENTIAL
JOHNSON CONTROLS
Watchdog Circuit
The Watchdog circuit performs two functions as fol-
lows:
• Power failure detection
• Program latch-up detection/prevention
The Watchdog Circuit monitors the +5 VDC from the
power supply and the +3.3 VDC from the onboard reg-
ulator to determine when a power failure is occurring.
If the +5 VDC decreases to the threshold of (+4.44
VDC to +4.68 VDC) or the +3.3 VDC decreases to the
threshold of (+2.84 VDC to +3.04 VDC), a reset is is-
sued to the microprocessor and the chiller shuts down.
When power is restored, the white screen is displayed
and the boot-up is performed as described above.
When the graphic screen is displayed, the message
CONTROL PANEL POWER FAILURE is displayed.
The Watchdog Circuit also ensures that the entire pro-
gram is being executed and that the program has not
latched-up, bypassing important safety checks. The
Watchdog circuit is a timer that times-out if not given
a reset pulse within its time-out period (1 to 1.204 sec-
onds).
To prevent a time-out, the microprocessor sends a reset
pulse to the Watchdog circuit every time the complete
program has been executed. Since it takes less than 1
second to perform the entire program, the Watchdog
circuit doesn’t time-out under normal operation. How-
ever, if the entire program is not executed or something
prevents the microprocessor from sending the reset
pulse as described below, the Watchdog circuit times-
out and sends a reset to the microprocessor, initiating a
re-boot. If running, the chiller shuts down. The display
momentarily blanks and the white screen is displayed
while the boot-up program executes as described
above. When the graphic screen is displayed, either of
two messages is displayed depending on the type of
Watchdog shutdown as explained below.
There are two different watchdog initiated shutdowns;
a HARDWARE watchdog initiated shutdown and a
SOFTWARE watchdog initiated shutdown.
In the HARDWARE watchdog initiated shutdown, a
program problem, on-board noise or hardware problem
could prevent the watchdog time-out. If this occurs, a
re-boot is initiated and when the graphic screen is dis-
played, CONTROL PANEL – POWER FAILURE is
displayed.
In the SOFTWARE watchdog initiated shutdown, the
program intentionally initiates the reboot because it
has detected program interruption. After the reboot,
WATCHDOG – SOFTWARE REBOOT is displayed
on the graphic screen.
Program Jumpers/Program Switches
The Program Jumpers and Program Switches are used
to alter the program operation or configure the mi-
croboard hardware for specific operation. This allows
the Program and Microboard to be universal for all
standard applications. The position of some jumpers
can be determined by the Service technician to meet
the desired operation. Others must be positioned ac-
cording to the requirements of the size, type or style
of components and thus are determined by the YORK
factory. The jumpers are plastic sleeves with metal in-
serts that are inserted over 2-prong or 3-prong conduc-
tors. The Program Switches are miniature switches that
are placed in either the ON or OFF position.
