Keithley 7020 Instruction Manual - page 36
Service Information
4-4
4.9
Input channel control
The mainframe reads digital input channels of the I/O card
from a serial, fi e-byte (40-bits) data stream (via INDATA
line).
Digital inputs are applied in a parallel fashion to the fi e in-
put channel registers (8 channels per register). When the dig-
ital inputs are read, the STROBE line goes high to latch the
input channel information. The INCLOCK line then clocks
out the information as a serial, fi e-byte (40-bits) data stream
(via INDATA line) to the mainframe. As data empties from
the lead register (U100), it is replaced by data via the Q7 line
of the registers down the chain.
Note that a channel is on (closed) when the input is high.
4.10 Power-on safeguard
NOTE
The power-on safeguard circuit discussed
below is actually located on the digital
board in the mainframe.
A power-on safeguard circuit, made up of a D-type flip-fl
and associated components ensures that output channels do
not randomly turn on (close) when power is cycled. This cir-
cuit disables all output channels during power-up and power-
down periods.
The PRESET line on the D-type flip-flo is controlled by the
68302 microprocessor, while the CLK line of the D-type
flip-flo is controlled by a VIA port line on the 68302 proces-
sor. The Q output of the flip-flo drives each I/O card output
channel driver IC enable pin (U109-U113, pin 8).
When the 68302 microprocessor is in the reset mode, the
flip-flo PRESET line is held low, and Q out immediately
goes high, disabling all output channels (output channel IC
enable pins are high). After the reset condition elapses
(~200msec), PRESET goes high while Q out stays high.
When the first valid STROBE pulse occurs, a low logic level
is clocked into the D-type flip-flop setting Q out low and en-
abling all output channel drivers simultaneously. Note that Q
out stays low, (enabling output channel drivers) until the
68302 processor goes into a reset condition.
4.11 Troubleshooting
Troubleshooting equipment
Table 4-1 summarizes recommended equipment for trouble-
shooting the Model 7020.
4.12 Troubleshooting access
In order to gain access to the scanner board top surface to
measure voltages under actual operating conditions, perform
the following steps:
1. Disconnect the mass terminal card from the scanner
board card.
2. Remove the mainframe cover.
3. Install the scanner board card in slot 1.
4. Turn on mainframe power to measure voltages (see fol-
lowing paragraph).
4.13 Troubleshooting procedure
Table 4-2 summarizes I/O card troubleshooting.
WARNING
Lethal voltages are present within main-
frame. Some of the procedures may ex-
pose you to hazardous voltages. Observe
standard safety precautions for dealing
with live circuits. Failure to do so could
result in personal injury or death.
CAUTION
To avoid contamination, always handle
the card only by the handle and side
edges. Do not touch edge connectors,
board surfaces, or components on the
card. Also, do not touch areas adjacent
to electrical contacts on connectors.
Table 4-1
Recommended troubleshooting equipment
Description
Manufacturer
and model
Application
Multimeter
Oscilloscope
Keithley 2000
TEK 2243
Measure DC voltages
View logic waveforms