JPM MPS2, Руководство пользователя

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9
MULTIPLEXING -TECHNICAL DESCRIPTION
The control of the multiplexing transistors is performed by an 8255 peripheral interface element (IC 25). The
CF70000 communicates with the main processor (IC 13) via an eight bit parallel bus BD0 to BD7 pins 27 to 34
respectively. The device is memory mapped and is enabled by Sel 3.
On reset the device is disabled since I/O enable (IC25 pin 35), is high. This sets the device into the input mode
and all outputs are pulled down internally to logic zero. This disables the column transistors and therefore all
lamps are extinguished.
During operation the data for the lamp/display system is set on ports A0-A7 and B0-B7. The specific row (one
of sixteen) is set up as a four bit code on port CO-C3. A 74HC4514 is used as a four to sixteen line decoder, to
produce a specific row select. By using this method it is impossible to set up more than one row at any one time.
Because in a fault condition, it would be possible to stop incrementing the row code, and hence leave a row
permanently on and hence developing 50 volts across a 12v lamp, a counter within ULA2 is used to disable the
drives if the row code stops incrementing.
In this instance the counter input (IC 11 pin 9) will be reset by the least significant row address C0 and will
consequently timeout. This will set the output (IC 11 pin 1) to the high state which will set all outputs trom the
74HC4514 low. This in turn will set all the row transistors off and disable all outputs.
A multiplex timeout will be indicated by the bottom right hand segment of LED 5 (7 segment) extinguishing.
The outputs trom the 74HC4514 CMOS decoder are interfaced to octal transistor drives (IC34 and IC36),
whose outputs are used to drive the sixteen NPN row transistors TIPL 790's (Q17 to Q32). These transistors are
capable of sinking 8A at a 1/16 duty cycle and an ambient temperature of +60°C which allows a complete row
to be on permanently.
The maximum mean current through a row transistor is as follows:
Assuming a lamp is 1.2w then at 12v the resistance is 120R. Thus at 50v the current will be 400mA. For a
complete row illuminated therefore this current will be 6.5A. To prevent excessive heat being generated in the
transistors the devices chosen have a very low saturation voltage, typically 1.5v maximum.
The column data set up on ports A0-A7 and B0-B7 are connected to octal transistor drives (IC17 IC18) which
drive sixteen PNP column transistors (Q1 - Q16) TIP 126. A 2R ½ w resistor is used in the emitter, tor the
following reason. Firstly, on switch on of a cold lamp, the current flowing would be very high since the lamp
would inhibit almost no resistance.
The 2R2 resistor effectively limits the current to 30A. This current is still excessive, however due to the flow of
this current through the resistor, the emitter would drop below that of the base and hence the transistor will tend
to turn off, and limit the current in itself. As the lamp warms up, the current will reduce, the drop across the
resistor will reduce and the transistor will tend to turn back on. Thus the 2R2 resistors (R1 - R16) provide a
"Cold Start" for the multiplexing system, preventing transistor breakdown.