94 34980A Service Guide
4 Troubleshooting and Diagnostics
34921A 40-Channel Armature Multiplexer with Low Thermal Offset
The 34921A 40-Channel Armature Multiplexer (40-Ch Arm MUX) is divided into two
banks with 20 latching armature switches (channels 1- 20 and 21- 40) in each. This
module also offers four additional fused relays (channels 41- 44) for making AC and DC
current measurements with the internal DMM with no external shunts needed. These
current channels feature “make-before-break” connections to ensure continuous current
flow when switching from one current channel to another. The current fuses are
replaceable.
This module also contains nine armature Analog Bus relays (channels 911- 914,
921-924, and 931), four on each bank that can connect the bank relays to the system
Analog Buses and one that connects the current relays to the current input of the DMM.
Through ABus1 and ABus2 you can connect any of the channels to the internal DMM
for voltage or resistance measurements. Refer to the simplified schematic below.
For the 34921A, relay and fuse part numbers are given on page 128 and the component
locator is shown on page 138. The table below shows the relationship of channel
numbers to relay numbers.
Bank 1 Bank 2
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