Knowledge Base

The 4 dedicated input-only pins on the MS120 are able to be set to work as NPN or PNP inputs. There is 2-position configuration jumper (link) that sets the mode. All 4 inputs are set by the one jumper, so you cannot mix modes.

The picture above shows the jumper in the NPN position and a switch wired into the input in NPN mode. In NPN mode the switch must complete a connection between the input pin and circuit common (0V) for the input to be read as ON. The yellow arrow indicates the position of the selection jumper. The input current is

I(in) = – V(supply) / 10000

The leading minus sign simply means the current is flowing out of the pin. The 10000 represents the 10K input pullup resistance.

The picture above shows the jumper in the PNP position and a switch wired into the input in PNP mode. In PNP mode the switch must complete a connection between the input pin and the power supply positive terminal for the input to be read as ON. The input current is

I(in) = V(in) / 10000

V(in) is the externally applied voltage, which normally will be equal to the supply voltage. The 10000 represents the 10K input pullup resistance.

The final picture, above, shows a PNP type sensor connected to the MS120. The sensor is powered off the input connector. ‘+’ on the sensor is it’s positive power Please refer to the sensor manufacturer’s technical data for the proper connections on the sensor.

Why have 2 different ways of doing it?

The reason we have 2 different ways are largely historical. NPN and PNP refer to the two basic types of bipolar transistor. Sensor manufacturers usually make sensors in both polarities, or sometimes they make sensors that can be used as either. It tends to cost less to make a NPN outputs (like our standard transistor outputs on SPLat) because modern bipolar transistor technology is biased in favor of NPN.

Which way is best?

Which of the two is best depends on your point of view. We have a preference for NPN because it makes designs a little easier and less expensive to make, so we can bring you more functionality per dollar. On the other hand, it is argued in some quarters that PNP is safer because an accidental short circuit to ground (the most probably kind of accidental short) is less likely to produce a spurious input. The validity of this argument will depend on the specific application.

We suggest that unless you have good reason otherwise, you use the NPN mode. This will improve your chances of making a control design that is capable of transferring to our other controllers (which at time of writing all use NPN).

Threshold voltages

The actual threshold voltage is nominally about 4.6V in either mode (min/max = 3/7) . In NPN mode anything less than the threshold is On. In PNP mode anything greater than the threshold is On. The processor knows the mode because the mode jumper is fed into an auxiliary processor input.

How to blow up the dedicated inputs
  1. Apply more than ±35V steady state for more than 10s to any one input.
  2. Apply more than ±100V momentarily