Chapter 2 NI2501/2503 Operation
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National Instruments Corporation 2-5 NI 2501/2503 User Manual

Four-Wire Mode

Four-wire mode is typically used for resistance measurements. In this
mode, CHxA+, CHxA–, CHxB+, and CHxB–, are switched together to
COMxA+, COMxA–, COMxB+, and COMxB–.

Matrix Mode

By adding some external wiring, you can configure the switch card as
atwo-wire 6 x 4 crosspoint matrix. (See Figure 1-4 in Chapter 1.) The
TB-2606 terminal block simplifies your wiring task by connecting the
necessary pins on the switch card’s front connector to create the 6 x 4
matrix.
In the matrix configuration, you can connect any ROWx signal to any COLx
signal.
Switch Control Circuitry
The switch control circuitry (SCC) is responsible for opening and closing
FET switches on the NI 2501 or relays on the NI 2503. You can load
commands for the SCC into memory storage using the driver software
you received with your kit. Consult the online help or your software
documentation for specific information on the appropriate commands.
There are two reasons for storing the commands in memory before the
SCC can process them:
• The operate/release time for the NI2503 is 3 to 5 ms. Using memory
storage, the software can send multiple commands to the SCC without
having to wait for a relay action to complete.
• The memory is used to store a scan list necessary for hardware random
scanning.
Random Scanning
The NI25 01/2503 can perform random scanning. In random scanning,
the switch card can scan the channels in any order.
The scan list is downloaded to onboard memory. Commands in the scan
listcan:
• Open or close relays
• Wait for an external trigger