PLC to PLC I/O

[Tom Jenkins]

Another vote for "interlock" as the description. I've done it direct PLC to PLC in the past when I wanted a fault in one machine to cause the standby to start. I used relay outputs on one PLC direct to the inputs on another PLC.

I don't see any need special need to use a relay for this connection. It really isn't any different than connecting the PLC input to an external switch. If the environment or voltage mismatch doesn't justify an interposing relay for all field switches it doesn't justify it for this kind of connection.

 

[MarkTTU]

I'll vote 'interlock' as well; especially for maintained signals...

We do this type of thing on all our products (heater OEM). We provide a digital input for START, STOP, and RESET and a digital output for FLAME ON, and ERROR; we also provide for analog output of process variable and an analog input for remote set-point.

Doing things this way allows our customers to connect their master plant PLC to our equipment without needing to know anything special about our equipment. For those customers that specifically want to talk to our equipment via a network we provide a 10/100 Ethernet port and a list of memory addresses; to date only one customer has used this.

 

[bernie_carlton]

We always provide interposing relays for foreign I/O signals. (Come to think of it we don't have a consistent internal company name for these signals.) We check the required voltage for signals to us (24VDC or 110 VAC). We provide a dry contact set (NO, Swinger, NC) for each output signal. Of course we wire the coil wires of the incoming signals and the contact wires of the outgoing signals using yellow wire (unless a different color is required by the customer). On the schematic the coil connections for incoming signals and the contact connetions for outgoing signals are represented so as to emphasize that they are free of all electrical connections to the local machine's wiring.

 

[elevmike]

Hello,

I was wondering if any of you ever wire the output of a plc to an input of another plc. This would be done with stand alone plc driven machines using AB micrologix controllers and without a network.

Note: I didnt read all the posts yet, but a quick answer is YES. Schems for doing this are as plentyfull as stars in the sky, but I used a 6 wire binay signal to communicate up to 32 differnt (pre-determined) messages to the other PLC. This required 8 inputs & 8 outputs on each PLC. The other 2 I/O were for send & recive confirmations.

 

[MarkTTU]

I used a 6 wire binay signal to communicate up to 32 differnt (pre-determined) messages to the other PLC.

Very clever; I'd never thought of doing it that way...

 

[TConnolly]

I also call them interlocks.

I've done this to interlock some AB and Modicon PLCs. There are four interposing relays used for inputs to the other PLC, allowing me to send/receive 16 messages.

The reason I used relays -vs- direct wiring is that I can place the relays together behind a barrier and mark that barrier to indicate that there may be voltage present there even when the power is off to the local cabinet.

 

[mellis]

What's wrong with the title of the original message?
"PLC to PLC I/O"

It's clear, concise and accurate. We all knew what he was talking about, right?

While these signals may indeed function as interlocks, they certainly don't have to be used in that manner. Mike's example of using a group of digital I/O as a message number is an excellent example. IMO, using the word "interlocks" for this catagory of signals can be misleading. Interface is a better term, and some people refer to them as "handshakes" which is not a bad term either. However, these terms need further qualification to be clear, there are many types of "interface".

I generally use interposing relays as well because I don't have to bother with pull-up resistors on 24vdc or loading resistors on 120vac. It also helps keep those foreign voltages separated.

 

[MarkTTU]

What's wrong with the title of the original message?
"PLC to PLC I/O"

Good point. I knew exactly what the thread was about before I clicked on it...

 

[OkiePC]

I prefer not to use relays unless absolutely required. When connecting these I/O, make sure your power sources' commons are at the same potential. I once connected a PLC5 ac output to a SLC ac input and blew up the power supply in the slc because the control transformer in the PLC came from phase 1 and phase 2 of a 3 phase supply and the SLC got it's power from phase 2 and phase 3. That put 240vac potential between the 120vac at the PLC and the neutral on the SLC. I had already wired several machines this way without problems until I ran across this one where someone had moved the input for the control transformer. Lesson learned.

 

[Binaural]

If the signal has to travel a long way, be aware that power supplies in a distant part of the factory or in a separate building may have a significantly different ground potential. For example, a 24v DC output from a PLC to another may be only 19V with respect to the local 0V reference by the the time it gets to the input on the external (not considering voltage drop either). This can be a little borderline for some input cards.

Basically I am echoing the good advice of many here - if you design anything for integration into a larger system, use interpolating relays

 

[BobB]

I generally use interposing relays as well because I don't have to bother with pull-up resistors on 24vdc or loading resistors on 120vac. It also helps keep those foreign voltages separated.

Yes. Normally go this way too because each PLC generally has it's own power supply. Add to that they are normally 24VDC systems off batteries and I use 24VDC/24VDC switchmode power supplies and the voltages "float". You can almost guarantee they are floating at different levels.

It is generally unwise to ground output negatives from these switchmode power supplies as you lose your isolation straight away.