Hi. I have been iterating different options for PLC-level digital inputs and outputs. If you have good practices in mind, I appreciate to hear about them. It does not matter if those are related to 5VDC symmetric signal output, or 24VDC OC output, or something else (except relay).

A

And for those who ask the question "can't help you without more specific specs" I answer that there are no specs or requirements. This is more about learning about what kind of solutions there are inside of "real" PLCs.

 

For Digital inputs we use LM339 (quad) or LM393 (dual) open collector comparators. Input resistors are large to limit input current.
For digital outputs we used bipolar high side switch with D-PAK transistors.


It is entirely possible that a similar MOSFET circuit would work as well and cost less to boot.

 

For Digital inputs we use LM339 (quad) or LM393 (dual) open collector comparators.
For digital outputs we used bipolar high side switch with D-PAK transistors.

Nice! Thanks.

 

I tend to use Opto-isolators on inputs and Open drain Mosfets on the outputs, or a relay

 

exactly.... isolation is important. you do not want single fault or wiring mistake to to take down multiple pieces of equipment. also some of them will use different polarity IO. i prefer to not be constrained or having to add different type of IO module.

 

At one time, for custom PLC type applications, I used the Opto-22 I/O isolated modules, They come in various configurations, 5v & 24v, with AC & DC outputs which had LED indicators and fusing etc.

e.g. of just two types.

 

At one time, for custom PLC type applications, I used the Opto-22 I/O isolated modules.

These are compact, but they are relays, right...

 

Not really, they are SS modules,
This is typical output style, later ones had both LED and fusing on the module itself..

Triac output:

 

Ai, see. There are both of them. This is the a relay version of Opto 22 module:

 

They have several versions of both Input & Output.
They also have footprints/dimensions for custom made boards if needed.