4-8 Troubleshooting Model 2750 Multimeter/Switch System Service Manual
Memory circuits
ROMs U156 and U157 store the firmware code for instrument operation. U157 stores the
D0-D7 bits of each data word, and U156 stores the D8-D15 bits. RAMs U151, U152,
U164, and U193 provide battery backed operating storage.
Semi-permanent storage facilities include NVRAM U136. This IC stores such information
as instrument setup and calibration constants. Data transmission from this device is done
in a serial fashion.
U194, U171, Y103, and BT100 make up the battery watchdog control, along with the real-
time clock. U171 automatically senses when the +5VD supply is being powered down and
then switches to BT100 for power.
RS-232 interface
Serial data transmission and reception is performed by the TXD and RXD lines of the
MPU. U159 provides the necessary voltage level conversion for the RS-232 interface port.
IEEE-488 interface
U158, U160, and U161 make up the IEEE-488 interface. U158, a 9914A GPIA, takes care
of routine bus overhead such as handshaking, while U160 and U161 provide the necessary
buffering and drive capabilities.
Trigger circuits
Buffering for Trigger Link input and output is performed by U146. Trigger input and out-
put is controlled by the TP5 and PC2 lines of the MPU.
Digital I/O
U146, U191, and U192 make up the digital input circuitry. External triggering can occur
on J1006 or J1007. U192 allows hardware handshaking to external controllers by gating
off triggers.
U188 and U189 provide digital output. The two driver ICs, U188 and U189, provide cur-
rent sink capability of 250mA each.
Module slot control
U122, U127, U170, U173, U174, U195, and U196 make up the control circuitry that
allows communication of relay data to Slot 1, Slot 2, Slot 3, Slot 4, or Slot 5.
Line sync
U179, U180, U181, U182, U183, U184, and U186 are used to control A/D triggers syn-
chronized at the zero cross point of the power line voltage. U186 controls zero crossing
detection, while U182 and U183 preserve the trigger states until the zero crossing thresh-
old is detected.
To Next Page
Loading...
Need help?
General
