Digital Controller Troubleshooting; In-Guard Microcomputer System - Fluke 8842A Instruction Manual

TEST 28: External Program Memory (U222)
A two-byte check sum is calculated over the entire 4K External Program Memory and
compared with the checksum bytes at the end of that memory. A special add and shift
algorithm minimizes the possibility of double errors cancelling. If something is wrong
with the External Program Memory, ERROR 28 is displayed.
TEST 29: Calibration Memory (U220)
Numerous single-byte checksums are placed in the Calibration Memory, one at the end of
each group of calibration constants. They are calculated in such a way that the single-byte
sum of all bytes in the Calibration Memory add to zero (all carries discarded) and the
single-byte sums of each group also add to zero. A new checksum is calculated and
written to Calibration Memory each time a full or partial calibration is performed. If the
Calibration Memory is not properly configured or not working correctly, ERROR 29 is
displayed. The accuracy of the 8842A is suspect.

6-40. Digital Controller Troubleshooting

The basic strategy in troubleshooting the Digital Controller circuit is to check first
whether the In-Guard Microcomputer ( C) system is functional, starting with the In-
Guard C itself (U202). Most of this circuitry is tested using the specially provided In-
Guard Troubleshooting Mode.
If the In-Guard C system proves to be functional, then basic instrument control is
assured and troubleshooting efforts can proceed in one of two directions. If the display
and keyboard appear to be malfunctioning, then they should be checked next. (See
Display System, below.) If the display and keyboard are functioning correctly, you can
omit Display System troubleshooting and proceed to verify that signals are arriving
correctly at the inputs of the analog control devices. (See Analog Control Signals, below.)
If these are also correct, the digital controller is functioning correctly, and you can
proceed to the appropriate analog troubleshooting procedure.
For the convenience of the following tests, Options -05 and -09 should be
removed if present. They should only be removed in the power-off
condition.

6-41. IN-GUARD MICROCOMPUTER SYSTEM

This procedure is performed entirely in the In-Guard Troubleshooting Mode. This mode
is established by shorting TP205 (U202-38) to Reference Low (TP306, or the L-shaped
shield around U202) prior to turning on the instrument. Refer to Figure 6-9. To maintain
this mode, the short must remain in effect after the instrument is turned on. When this is
done, the C programs U202-38 as an input (it is normally an output) to preclude any
possibility of damage due to the short.
To avoid damaging the C, the short must be initiated before
the instrument is turned on, not after.
The In-Guard Troubleshooting Mode also programs all the normal port outputs to display
a 1 kHz square wave except that the IEEE-488 output (U202-4) sends the word "55"
repeatedly at a rate of 2,000 words/second and the A/D trigger (U202-40) is a square
wave at its normal frequency of 80 Hz, and DM and P23 stay high. (The data received at
U202-5 is meaningless.) Adjacent port outputs display opposite phases of the 1 kHz
square wave. All C pins that are normally only programmed as inputs are also
NOTE
CAUTION
Maintenance
TROUBLESHOOTING
6
6-45