National Instruments 321906B-01 用户手册

Chapter 2

NI 2501/2503 Operation

2-4

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 National Instruments Corporation

The relay multiplexer consists of four banks of six channels. The switches 
for the six channels in each bank connect the channels to a common output 
for each bank.

•

Bank 0 consists of Channels 0–5 and Common 0.

•

Bank 1 consists of Channels 6–11 and Common 1.

•

Bank 2 consists of Channels 12–17 and Common 2.

•

Bank 3 consists of Channels 18–23 and Common 3.

Each channel connects to the corresponding bank common by closing its 
relay/FET switch. You can have multiple channels connected to the bank 
common at the same time. Bank connection relay/FET switches can 
combine the commons of the four banks. See Appendix D, 

, for illustrations of the NI 2501 and NI 2503 architecture.

The NI 2503 relays are single-side stable. When power is removed from the 
relay coil, the relay returns to its de-energized state. The de-energized state 
opens all the relays, disconnecting the device from external inputs and 
outputs.

See Figures 1-1 through 1-4 in Chapter 1, 

, for the pin names used for the different modes of operation. 

Different names are used for the pin assignments in the different modes to 
better reflect the operation in the different modes and to simplify the task 
of wiring.

Two-wire mode is the default and most common configuration for the 
NI 2501/2503 switch cards. The CHx+ and CHx– channels are switched 
together to the COMx+ and COMx– signals for the corresponding bank.

One-wire mode doubles the channel count of what is available in two-wire 
mode. However, the signals connected to the switch card need a common 
reference. For one-wire measurements, Bank 0 includes extra relays 
between the multiplexed channel relays and COM0. The 1WIRE relay 
switch connects COM0– to the 1-Wire Lo Ref signal. The HLSELECT 
relay connects COM0+ to either CHx+ or CHx–. The de-energized states 
for these relays configure the device for two-wire measurements.