analog as a digital input MLX1100

Hello all,

Can I use my two onboard analog inputs as digital inputs without having to use a voltage divider circuit to drop my 24vdc to 10vdc. The specs manual is not very clear about the maximum voltage I can place at my analog input terminals, it states "voltage input over voltage protection - 10.5VDC" The client requires an upgrade of 2 x DI but there is not enough room in the cabinet. thanks!

There is on-board clamping (zener diodes) that will limit the analog input presented to the A/D converters to 10.5 V.

The protection is intended for short-term (spike) suppression. Applying 24V to these inputs for long periods will stress the components beyond their design capabilities.

I would, (if I had to use analog inputs as digital inputs), just use a simple 2R,R potential divider network, giving me 0V (off state), and 8V (on state).

But i would also latch my logic state with simple comparison instructions, e.g. GRT and LES, would give me a logic "1" if the input was above 6V, and a logic "0" if the input was below 2V.

This would filter out any noise on the analog inputs.

HTH

thank you for your response, I will take your advice and use a voltage divider circuit.

welcome

The 1100 supports expansion I/O:
http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/5865007/5865012/tab2.html

In my opinion this sort of kludge fix is appropriate only for emergency short term use, and not for a new system. Any cost savings is quickly negatged by extra programming time, testing, and future confusion when the next guy has to trouble shoot the system.

The discrete input card is only $163 - go for it.
http://raise.rockwellautomation.com...ents.asp?CID=DA05C37B6D4C4518B0CF741741816F83