Chapter 2 NI2501/2503 Operation
NI 2501/2503 User Manual 2-4
©
National Instruments Corporation
Multiplexer
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 theco mmons of the four banks. See App e ndix D, Architecture
Drawings, for illustrations of the NI 2501 and NI2503 architecture.
The NI2503 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 through1-4 in Chapter 1, Routing Signals with Your
NI25 01/2503, 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

Two-wire mode is the default and most common configuration for the
NI2 501/250 3 switch cards. The CHx+ and CHx– channels are switched
together to the COMx+ and COMx– signals for the corresponding bank.

One-Wire Mode

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.