A & A Scales PT300, Service Manual, Page: 16 / 52

PT300, Service Manual
Rev B, October, 2000
15
Display Driver:
Any LCD display must be driven by a waveform with a net zero DC
voltage. If the driving voltage has any DC component the display will be
destroyed. A display driver chip (U10) provides the correct drive for 4 of
the display digits. The leading 100000's digit, the trailing 0/5 digit and all
annunciators are driven by U6, U7, and U8. U6 latches most of the
desired segment drive information. One line of U6 drives the serial output
circuitry. The low battery line is driven directly by the voltage sensor.
The EXCLUSIVE OR gates (U7 & U8) drive LCD segments with a signal
in phase with the backplane for off segments, and out of phase for on
segments.
O ff O n
Figure 6 LCD Drive
Serial IN/OUT:
U15 Reads a 20 mA current loop and converts this to an opto-isolated
TTL signal across R31. R29 and R30 act as a voltage divider to limit the
current through the current loop receiver diode. D17 provides a path for
reverse voltage spikes. U13 is programmed to output a regular 4800 Hz
pulse on pin 17, this is used as the source of non-maskable interrupt.
This timebase pulse is used to form a UART in software. U16 converts
TTL signals into either RS-232 or a 20 mA current loop signal. R34 acts
to limit the current in U16. D18 cancels the Vbe drop of Q4, allowing
current through R38 until its voltage drop matches D19 (0.7 Volt). This
results in 20 mA across the collector-emitter of Q4. When used as a RS-
232 transmitter, OIL- is grounded and the output is taken from OIL+. For
current loop, output is taken from OIL- and ground is taken on the other
side of the receiver logic.