4-8 Troubleshooting
RS-232 interface
Serial data transmission and reception is performed by the TXDB and RXDB 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 output
is controlled by the IRQ4 and PB3 lines of the MPU. U164 provides additional logic for the
trigger input to minimize MPU control overhead.
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.
U122, U188, and U189 provide digital output. U122 is a shift register that feeds updated
output information to the two driver ICs, U188 and U189, which provide current sink capabil-
ity of 100mA each.
Module slot control
U127, U173, and U174 make up the control circuitry that allows communication of relay
data to Slot 1 or Slot 2.
Line sync
U179, U180, U181, U182, U183, and U186 are used to control A/D triggers synchronized 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 threshold is detected.
Analog circuitry
Refer to Figure 4-3 for the following discussion on analog circuitry.
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