F&G Analog Input as Digital Input


Thread Starter


In a Fire and Gas System we are utilizing AI board but that will act as DI for the signals coming from Manual Call Points. In the circuit we are including 2 resistors, one in series with the contact (R1) and the other in parallel with the contact (R2).

<b>What is the main purpose of these two resistors and why not using a normal DI board to monitor the signals?</b>

They are used to monitor the line. The resistor values are known.

If the line is open, a wire came off, etc., the signal will be zero.

If the contact is closed the signal will the result of the one series resistor.

If the contact is open the signal will be the result of the two resistors.

Good luck,

Bruce Durdle

You have a monitored circuit, with a resistance in series with the supply and a terminating resistance at the remote end. If there is a circuit failure (either open- or short-circuit) there will be a predetermined change in voltage.

Typically, for a 24 V supply, the monitored voltage will be about 20 V. Loss of supply, or a wire short to ground, will give 0 V. Operation of one or more of the monitored points will cause the monitored voltage to drop by a measured amount.

You don't say what the field device is, but gas detectors frequently have analog outputs.

There's always some shunt resistor in parallel across the analog input's (+) and (-) so that a voltage drop is developed for current loops.

Are you sure that the series component is a resistor? A diode is sometimes installed forward biased in a current loop so that a millimeter placed in parallel across the diode can read the loop current without breaking the circuit open to install the millimeter.

See an example of diode insertion here:
The AI board we are using has a shunt stick that can either work on current or voltage. Normally in the designs I've seen they are working with current. But I guess this depends on the field device right?

Bruce Durdle

You really need to check with the equipment supplier or system designer - this is a critical safety element and one you can't afford to get wrong. Depending on the detail of your local safety codes, they should give full details of the system to allow it to be operated and maintained safely. Maintenance details will include for example details of how the system can be tested without overriding or possibly disabling these essential safety features.

If for some reason there is no assistance available from this quarter, you need to research the system you've got in depth and make sure you understand exactly how it is supposed to work. Then make sure it is fully documented so that no-one makes well-intentioned but wrong modifications to it, and that maintenance/check procedures are valid.

If you think that you're out of your depth in this area, you need to make that plain to whoever is expecting you to deal with this problem. (Unfortunately, there is a lot of: "This is an electrical problem - you are supposed to be an electrician - so you MUST be able to deal with it" out there!)