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.

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.

Using program commands or the mainframe front panel, you can control each of the channel switches individually, and thus configure the 34921A module in these modes:

• Two independent 20-channel 2-wire MUXes. This configuration requires neither using external wiring nor connecting through the internal Analog Buses
• One 20-channel 4-wire MUX. This configuration requires neither using external wiring nor connecting through the internal Analog Buses. For 4-wire resistance measurements, the instrument automatically pairs channel n on Bank 1 (source) with channel n+20 on Bank 2 (sense) to provide these connections. Four-wire controls occur only when doing 4-wire measurement operations through the internal DMM, or scanning a channel previously configured as 4-wire
• One 40-channel 2-wire MUX. You must use external wiring or connect through the internal Analog Bus relays for this configuration. For example, closing Analog Bus channels 913 and 923 connects Bank 1 and Bank 2 through ABus3. Or, externally you can connect COM1 to COM2 to create this configuration

Low thermal offset voltage makes the 34921A ideal for low-level signal switching. The 34921T optional terminal block provides a built-in thermocouple reference junction that helps minimize errors due to thermal offset when you measure thermocouples.

This module has capability to scan as many as 100 channels/second using the internal DMM. With the automatic “break-before-make” connection operation, you are assured that no two signals are connected to each other during a scan. When using the module in a non-scanning mode, you can close as many channels as you wish.

This module is safety interlock protected, which means whenever the D-sub connector end of the modules is exposed, the Analog Bus relays automatically open and disconnect from the Analog Bus.

When power is off, all channel relays maintain state, and the Analog Bus relays open.

For additional information on the 34921A, please see the document linked here beginning on page 25.

The chart below provides a brief overview of multiplexer modules for the 34980A.