S7-200 224XP - Analog as Digital?

[Davo]

Hi. I just took a Siemens class, but I can't find an answer in my book... or online. I have an S7-200 224XP with 10 discrete outputs, but I need 12. It seems like I should be able to send current through the analog ports to energize 24VDC relays. Can this be done? How, please?

[Crossbow]

Obviously didn't pay much attention in class. Analog inputs and discrete inputs are 2 completely different things. You can't just send current through the analog to make it turn on. For another thing, the analog inputs on that unit are voltage only (-10 to +10 volts DC). Analog inputs are variable signals, not on/off relays. Buy an I/O extension block.

[Davo]

Thanks, I get that... but why can't I vary it from 0 to "full"? If there's not enough voltage to power a relay, that's a different problem - we can use 5VDC relays if needed. Edited November 8, 2007 by Davo

[Crossbow]

You can't power a relay from an analog output. Buy the extension block. The analog output is not designed to carry a load like a relay coil.

[Davo]

Okay, thanks.