Service; Protective Control Time Constraints; Signal Converter; Watchdog Protection - McQuay MicroTech Unit Controller Installation And Maintenance Data

PROTECTIVE CONTROL TIME CONSTRAINTS

Operation of the mechanical low pressure switch (MLP), high
pressure switch (MHP), and oil pressure differential switch
(OD) serve to instantly shutdown an operating machine if
values in the open range of those shown in Table 12 occur.
Operation of motor overloads, and other safeties wired in
series with the MCR coils, and Surgeguard and Guardistor
relay operation also all result in instantaneous trip.
To prevent nuisance tripping from fluttering flow switches,
or delayed interlock closure, devices connected to terminals
7,8, 9, 10, 11, and 12 on TS1 terminal strip have a built in time
lag of 1.7 to 3.1 seconds prior to operation of a shutdown. Ob-
viously, if a motor overload trip occurs, the protection is in-
stantaneous, because the contacts are in the main MCR coil
circuits. The readout display is delayed because of the loca-
tion of the logic in the time delay circuit.

SIGNAL CONVERTER

The McQuay furnished signal converter accepts a zero to
5, 6, or 7 volt AC signal and converts it to a zero to 4 volt DC
signal.
If the converter should fail and pass AC volts to the
MicroTech terminal strip, TS4, the unit will trip off, and the
display will signal a "Mtr Current Fault".
If the signal converter fails "Open", the display will read
"Waiting High Amps".

WATCHDOG PROTECTION

The MicroTech panel includes two microprocessors, in the
Display EnGinn mounted on the keypad door, the other in the
logic cartridge mounted on the mother board.
Each of these two components has a blinking LED that
blinks independently of one another.
A third light marked "Microprocessor Status" on the keypad
door blinks in union with the control EnGinn LED silently pro-
claiming normal or troublesome operation to passersby.
The two microprocessor circuits are connected through the
daughter board to a protective circuit that keeps watch over
the operation of both.
In the event a malfunction occurs in either of the two
microprocessors, the watchdog circuit instantly removes
+5VDC power to the output board, thereby de-energizing all
outputs. This action immediately shuts down the chiller.
If the control EnGinn microprocessor fails, indicated by the
control EnGinn LED being constantly "on" or "off" but not
blinking, the chiller can not be run until the cause of the failure
is corrected.
In the event the problem is caused by the display EnGinn
microprocessor, indicated by the display EnGinn LED being
constantly "on" or "off" but not blinking, the control panel
may be de-energized, the watchdog circuit de-activated, and
the unit allowed to operate temporarily if necessary without
the display EnGinn operative.
To turn a chiller back on with a bad display EnGinn
microprocessor,
Trip the 3 amp circuit breaker, or otherwise de-energize
the control power to the panel.
Remove the watchdog jumper from the daughter board
(See Figure 27).
Disconnect the communications lead linking the two
microprocessors together. Unplug the lead at the Port A
connection on the inside of the door.

SERVICE

Re-apply the power to the control panel.
After power has been re-applied, the unit will start if it had
been programmed for remote control; but not without fur-
ther control refinement if the unit control was on "Local",
With the keypad and display initially programmed for
"Local" control, and the display EnGinn inactive because
of a malfunction, the keypad cannot be operated to start
the chiller. The display panel must be replaced immediate-
ly, or a portable computer with IBM capability and
MicroTech software connected at the communications Port
A on the control EnGinn.
Another option exists if the unit is equipped with a
telephone modem. The unit may be called over phone
company lines and instructions phoned via an IBM com-
patible and MicroTech software.
Because the avenues available to return a chiller to
operating status are more cumbersome if the keypad and
display are programmed for "Local" control, we recom-
mend that the unit be programmed for "Remote" and a
jumper or toggle switch wired across Field Connection ter-
minal strip terminals number 9 to 64 (or 65) if a conven-
tional remote switch is not planned.
Figure Daughter board showing Watchdog Jumper
27.
DAUGHTER BOARD GROUND WIRE DIODE
Daughter board terminal strip TS2 should have a diode, con-
nected across terminals 1 and 8. This diode is standard on
MicrcTech panels manufactured on or after April 1, 1989, serial
number 5UDXXXXX. Panels manufactured before April, 1989,
will not have this diode and it must be added during a routine
visit to the job.
This modification consists of installing a standard Surgegard
diode, part number 350A034H04, across terminals 1 and 8
on terminal strip TS2. To install the diode, remove jumper wire
605 from terminals 1 and 8. Connect the cathode (banded)
end of the diode to terminal 8 and the other end to terminal 1.
This modification will help prevent many potential daughter
board failures and prolong the useful life of !he board.
WATCH DOG
JUMPER
IM 403 I Page 35