Data Transfer of internal bits between 2 Micrologix 1400

Hello there,
I'm trying to send the state of a single internal bit (B3:3/8) of my main ML1400 PLC to a different ML1400 over Ethernet using the MSG instruction. But it seems like MSG takes whole words only (e.g.: B3:3).

Does anybody know if there is a different way?

Thanks

John93 said:

Hello there,
I'm trying to send the state of a single internal bit (B3:3/8) of my main ML1400 PLC to a different ML1400 over Ethernet using the MSG instruction. But it seems like MSG takes whole words only (e.g.: B3:3).

Does anybody know if there is a different way?

Thanks

Click to expand...

Other than a wire from a discrete output of the sender to a discrete input of the receiver, no.


Is there a problem if the second 1400 receives 16 bits but is only interested in one? As long as the communicants agree on the protocol i.e. where that one bit is in the word, any excess noise-bits in the message can be ignored.


Another option would be to write a 1 to an unused word of the B3:3/8 is 1 or write a 0 if not, and send that word, but that is basically the same approach.

Welcome to the PLCTalk forum community !

You should "pack" your boolean values into a 16-bit Word (B or N data table type) and send the whole word over the network.

A-B does not support bit-level reads and writes on purpose, for data integrity and network efficiency reasons.

That makes sense. I will do that.
It just felt like waste to use a whole word for a single bit.

Thanks both of you for your prompt reply.

The overhead involved in transporting a frame over Ethernet or over serial is much larger than the data payload itself. Since data is organized into bytes, the biggest packet difference between a Word and a Bit would be a byte.

Even when memory was precious in the PLC-2 platform, messaging was done word-by-word.

Since you'll be sending a word whose other bits are not significant, you can use one or more of them for a watchdog function, which can benefit most inter-system data transfers.