Agilent Technologies Welding System Agilent Technologies Welding System User Manual

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Chapter 4 Theory of Operation
Earth-Referenced Logic
90
Earth-Referenced Logic
Referring to the schematic shown on page 132, the earth referenced logic
circuits schematic provides all rear panel input/output capability.
Microprocessor U10 on the bottom board handles GPIB (IEEE-488) control
through bus interface chip U8 and bus receiver/driver chips U6 and U7 on the
bottom board. The RS-232 interface is also controlled through microprocessor
U10. RS-232 transceiver chip U12 on the bottom board provides the required
level shifting to approximate ±9 volt logic levels through on-chip charge-pump
power supplies using C11 and C12 on the bottom board. Communication
between the earth referenced logic interface circuits and the floating logic is
accomplished through an optically-isolated bi-directional serial interface.
Isolator U2 on the bottom board couples data from U10 to processor U17 on
the top board. Isolator U1 on the bottom board couples data from U17 on the
top board to microprocessor U10 on the bottom board.
Front Panel
Referring to the schematic shown on page 133, the front panel circuits consist
of vacuum fluorescent display control, display high voltage drivers, and
keyboard scanning. Communication between the front panel and floating logic
circuits is accomplished through a 4-wire bi-directional serial interface. The
main controller U17 can cause a hardware reset to front-panel controller by
signal IGFPRES. The front panel logic operates from -11.4 volts (logic 1) and -
16.4 volts (logic 0). The front panel logic high supply (-11.4 volts) is produced
by the -16.4 volts bias supply and the voltage regulator U2 on the front panel
board. The four serial communication signals are level shifted by the
comparator U8 from the floating logic 0 V to 5 V levels to the -16.4 V to -11.4 V
levels present on the front panel assembly. U6 acts as the serial shift register
interface for the front-panel controller U7 on the front panel board.
Display anode and grid voltages are +16.4 volts for an "on" segment and -16.4
volts for an "off" segment. The -10.2 V cathode bias for the display is provided
by filament winding center tap bias circuit VR3, R43, and C32 on the top board.
Keyboard scanning is accomplished through a conventional scanned row-
column key matrix. Keys are scanned by outputing data at front-panel
controller U7 port pins P0.0 through P0.3 to poll each key column for a key
press. Column read-back data are read by the microprocessor at port pins P1.0
through P1.3 for decoding and communication to the floating logic circuits.

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