Goals and Symptoms

Any device that needs to connected to digital inputs, for example between Scom and S8 as can be seen in the diagram below, needs to be able to handle 30Vdc across its terminals.

There are ways to manipulate the circuit so that a device that cannot withstand the full 30Vdc can be connected. Connecting directly however will not work.

Causes and Fixes

Question: What if the someone is using a device that can only stand 16.5Vdc?
In such a scenario a relay can be used to isolate the two circuits. When the digital output closes the switch on the left hand side, it only sees 12 V across it. When the circuit on the left is closed, a current will flow throw it hence forcing the right side of circuit to close and the meter to register a pulse. The meter will receive a pulse of 30 V and the width of that pulse would depend on how quickly the switch on the right hand side opens and closes.

Generally relays are more expensive to implement, and an electronic circuit could easily be set up in order to achieve the same result as above.