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The Siemend LOGO is a small simple logic module you could use. 24 DI 20DO 8AI 8AO. You can also network them to gain more I/O and it talks modbus tcp. It has a little display on it so you can change parametets without an HMI. We use these alot. The programming tool is good and in FBD but is has some limitations and is not as powerfull as a PLC programming tool. It is not scalable to the TIA portal and the s7-1200. If it's mainly yo replace relays it's perfect.

My single problem with the LOGO (and the same units with Idec's brand on them) is the memory.

I twice had to update a LOGO and quickly hit the MEMORY FULL and wanted to add more logic. I had to drop an input once and had to reduce the logic or features I was trying to add.

And I didn't think the programs were very big and could have written a program twice that size using the amount of IO it had.