Digital output in Normally Closed Configuration? - old forum post



Joined on 11-09-2004
Posts 2
Digital Output as Normally Closed

A simple question about digital output cards. I need the contact to act as a normally closed contact. Is there a way to do this without the aid of IOcontrol strategies? Or do I need to write the function into the code as always being turned on until the user turns off? Any help would be greatly appreciated.
08-10-2005, 3:09 AM

Joined on 12-01-2003
Optomation Systems, Madrid, Spain
Posts 153
Re: Digital Output as Normally Closed

An interesting question, usually ignored by most software engineers, but well worth considering for real world applications. While conventional relays can offer NO/NC (Normally Open/Normally Closed) contacts, optically isolated outputs such as OAC5 or SNAP-OAC5 are always equivalent to a NO circuit. I.e. on power loss they always default to high resistance (off or open circuit).

Actually its pretty obvious if you think about it as they are basically electrical circuits, which tend not to work too well without power! I wouldn’t recommend trying to solve the problem in software, power loss to the module or even response during control engine strategy startup or reboot will give the wrong results for your application. Instead use a NC output contact on a conventional relay and use the Opto output module to power the control coil of the relay.

Dont forget that the Opto digital output is volt free, so you need to power the circuit between Opto output and relay, typically at 24VDC which implies the use of a SNAP-ODC5-I or similar. Your application software then uses 0 to deenergise the relay coil, maintaining power in the relay output circuit and only needs to send a 1 to energise the relay coil, interrupting power in the relay output circuit, if required.

From the software point of view 0=OK while -1=Interlock/Disable. To avoid your relay output contacts burning out, use a suitable Amp rating for the relay, not only considering predicted startup current, but also bearing in mind that the maximum current rating is usually specified for AC voltage and will be much lower when used in a DC voltage application. Warning, not all people who sell relays actually know this!!