Data Acquisition (DAQ) and Control from Microstar Laboratories

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Q10028 Mounting termination resistors on boards

Tags: Help, DAP, termination, 4-20 mA, loading, balance, sensors

Applies to: All DAP models; MSTB009, MSTB010, MSXB037, MSXB086

My signals require a termination network. I originally planned to place the termination resistors at the connector, but with the high-density D-connectors on the ends of my termination boards, this is not a practical option. Is there a better way?

If you are using a MSXB086 4-20 mA receiver board, you do not need to worry about termination resistors. A high-precision loading and termination network is already present on the board.

If you are using MSTB009, MSTB010, or MSXB037 boards, these boards have a termination breadboarding area specifically intended for your purpose. There are through-hole pads adjacent to the connector terminals where you can mount the termination components.

breadboarding area on termination board

The image on the left shows an MSTB009 board, illuminated to make the traces as visible as possible, though this distorts the apparent colors. You can see part of the labels for the signals, in the sequence G0, S0, G1, S1, G2, S2, etc., matching up with the slots in the Wago quick-connect strips. The other board models have similar pads and traces.

The traces are easiest to see near the center, at roughly the G2 - S2 position. For each pair of quick connect terminals, there are three traces, each with multiple connection pads. In the center, a trace carrying the signal from the end connector extends out to make a T shape, with three connected through-hole pads below each side of the T. Upward and left from this, a trace extends from the G terminal of the quick connector to four through-hole pads. Upward and right, a trace extends up from the S terminal of the quick connector to another four through-hole pads.

It is easy to see on your termination board, but hard to see in this photo, that there is an additional trace that connects the upper right corner of the T to the connector pad just above it. It is obscured in the photo because of a printed X symbol directly on top of it. This trace completes the normal connection between the end connector signal pin and the quick-connect terminal for the signal. To put a component in series with the signal path, you will need to cut away this trace at the board surface — cutting any trace voids the warranty, but there is little risk, because the termination boards are tough and have few sensitive electronic components. If you later regret making this modification, it is a simple matter to attach a small jumper wire across the pads to restore the original connection.

Example: You have a single-ended input for a 4-20 mA current loop. To convert this signal into a voltage range within the -5V to +5V range limits, you need the current drive signal from the connector to pass through a load resistor to ground. You can mount this resistor in the solder pads on the left side. The link connecting to the input trace remains in place, routing the signal to the S terminal. You must have a return path for your current loop, so you can connect this return wire to one of the other ground-side pads on the left, and run this to the current loop supply return point.

Example: You have a differential voltage input and you need to give it a balanced 100K resistive loading to ground on each side as a balanced discharge path to ground. This involves a pair of inputs. Suppose these are signal pins marked as S2 and S3 on the termination board. These are the same signals that will be recognized as the differential pair D1 when you adjust your input sampling configuration for differential operation. You mount one 100K resistor in pads between the central signal trace and the ground trace on the left side of S2. Moving over one slot, you mount one 100K resistor in pads between the central signal trace and the ground trace on the left side of S3.


See the MSXB037 hardware manual for more information about the layout and usage of its termination pads, with application examples.