What Are the Optional Isolation Resistors of the TB-264x Series?
Hardware: Switches>>Multiplexer/Matrix>>PXI-2532
Problem: In the Installation Instructions of the TB-264x terminal block, it states "The NI TB-264x has ribbon cable headers to connect signals to the switch, and it provides optional isolation resistors to protect the reed relays from capacitive loads."
What does this mean? How can I tell if I have the resistors or if I don’t? What if I don’t want these resistors?
Solution:
TB-264x is a series of terminal blocks that configure the PXI-2532 high-density matrix into a number of different topologies. Each terminal block can be ordered with or without column protection. The protection resistors help to prevent inrush currents from capacitive loads which can easily damage/weld reed relays.
The version of the terminal block WITHOUT column protection will have 0 Ohm jumpers on the columns. For the version of the terminal block WITH column protection, the 0 ohm resistor in series with each column has been replaced by a 100 ohm isolation resistor. Since this is an optional feature, the two terminal block versions have different part numbers so that, at the time of purchase, you can select if you would like the TB model with the 100 Ohm column protection resistors.
Both versions of the terminal block will contain 100 Ohm protection resistors in series with the rows. Since this is an optional feature, you can bypass the row protection resistors using the 2x1 shunts, seen below, that are shipped with the terminal block.
The ribbon cable headers are isolated from the PXI-2532’s reed relays through these protective 100 Ω resistors. You can insert the 2x1 shunts in the J5 Row Protection Bypass Connector as seen in the images below.
You can tell if the Terminal Block you purchased has the column protection resistors by checking the part number (P/N or Assembly number) printed on the terminal block’s column connection board. For instance a part number like 191682X-02 (any number with a -02) will be the terminal block the 0 Ohm jumpers- no column protection. A part number like 191682X-01 (any number with a -01) is the terminal block with 100 Ohm isolation resistors.
A second method of determining if you have a terminal block with 100 Ohm Column protection is to measure the resistance from a row to column after closing that relay. Without the column protection, you could expect to see a path resistance of 1 or 2 Ohms, perhaps 4 to 5 Ohms if you have high cable resistance. However, if you have a terminal block with the 100 Ohm column protection, the path resistance would be 102 or 104 Ohms. NOTE: When measuring the row to column path resistance, keep in mind if you have the 100 Ohm Row protection in place or if you have bypassed the 100 Ohm Row resistance by inserting the jumpers.
Related Links:
Developer Zone: How to Connect Signals to the NI PXI-2532
Product Page: NI PXI-2532
Knowledge Base 4QQH8D9G: How to Connect the TB-264x's Columns to the SCB-264x
Attachments:
Report Date: 11/04/2008
Last Updated: 11/04/2008
Document ID: 4R3C2L9G
