TTI PL Series Service Manual page 12

the status signal on pin 2 is read by the microcontroller every 6ms. When a reading is ready the
microcontroller reads the 12 bit binary value and then converts it to 7 segment BCD and stores it
ready to be sent to the display. After each reading the microcontroller switches the input
multiplexer IC3 to the next required input. In this way it is possible to read and display any of the
following:
The multiplexer is controlled by IC1. The decision on what to measure and display at any time is
taken by the microcontroller and in order to do this correctly a number of status signals and
switches are monitored on a regular basis. These are:
These signals may be read by the microcontroller as required.
Service Notes
On Issue 3 and earlier main pcbs the current damping was provided by a digital filter in the
microcontroller program and R65 and C38 were not fitted. Also if jitter is experienced on the
meters near full scale, this can be overcome by increasing C1 to 330n. If the integrator capacitor
C4 has high dielectric absorption this will cause non zeroing of the current meter; this can be
overcome by simply replacing it even with the same type as the yield is very good. On later pcbs
more space has been provided to allow the larger pitch polypropylene type to be used.
Poor regulation can be caused by incorrect adjustment of the differential gain or the output is
oscillating.
Quad Mode Dual Switchbank Assembly
The Quad Mode Dual units have a bank of four switches which allow four modes of operation to
be obtained.
Isolated:
Parallel:
Series:
Tracking:
PL330QMD Master
The main pcb on the master side on these instruments has some extra circuitry which allows
current measurement above 4 amps and is only active when parallel mode is selected, QMPAR
going low. IC10 halves the reference input to the A to D and selects VOL02 to give the correct
output voltage readings. The current readings only read half and the microcontroller multiplies
the answer by 2 before driving the display. This results in a resolution of 2mA and not 10mA which
would be the case if a divide by 10 was used. See calibration section for correct adjustment of
VR10 and VR11.
Output Volts (equal to Preset Volts when the output switch is off)
Output Current
Preset Current
ILIMIT true if in current limit
OPON the output on/off switch signal
SW1 (Release) depressed. All interconnections between the two units is removed.
SW2 depressed. The Slave unit pass transistors are driven from Q1 on the Master
PCB, and their output is combined with that of the Master unit, thus doubling its
current capability. VR5 on the Master unit is disconnected from R47 and connected
to a network generating twice the control voltage. The slave unit becomes
inoperative.
SW2 depressed. The Slave unit positive terminal is connected to the Master unit
negative terminal.
SW4 depressed. As for Series, but additionally R36 on the +ve input to the Slave
unit voltage control op-amp IC7B is disconnected from IC7A and is instead
connected to a potential divider connected between Master unit +ve output and
Slave unit -ve output. This maintains the voltageage at the Slave +ve output (and
hence the Master -ve output) equal to ½ of the total output voltage, thus producing
tracking supplies of ± 30V controlled from VR1 and VR2 on the Master unit. VR1
and VR2 on the Slave unit become inoperative.
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