Block Transfers

Hello, all.

I am working with a PLC-5/20 and i am trying to set up block transfer instructions for dealing with the transfer of analog data to/from the PLC. I have two things that I'm stuck on that i was hoping i could receive some clarification on. I have read Ron's "mail man" explanation and it was helpful, but i am still a little unsure.

1. What exactly does the "Length" parameter in the BTW/BTR instruction represent? I originally thought it was related to the analog points that you were monitoring/controlling, but that does not seem to be the case. It sounds like there is more to it (scaling, etc.) I am trying to send data for 4 analog outputs from the processor to a 1771-OFE2 module. Can someone assist me in how to determine what my length should be?

2. I have also read something about block transfers needing to be used in pairs. So for my situation, i was originally thinking i only needed a BTW, but do i also need a BTR? If so, could someone again provide the reasoning for this? The instruction set help and the manuals that i have read through are just not quite getting it to click for me.


Thanks,

Justin

Typically they are used in pairs, for example. An analog input card.

Use a BTW to write the analog card configuration, this typically only needs to be done when a PLC first starts up. Or if some odd-ball error were to come up. There is no need to BTW the configuration each PLC scan because it should never change. However a BTW is still required to set the configuration (4-20mA, 0-10V, scaling..etc).

However, a BTR needs to always trigger because it is an analog input card and you need to constantly read the IO points.

With an analog output card the opposite is true however a BTR in this case is handy to read status information to watch for errors.

The length parameter will be the number of words you need to read/write. This is always different pending the device you are using, so pull out the manuals. You would see very clear tables of what the words/bits represent and the lengths required.

There are variations, which I won't get into but that should answer your basic questions.

Read the manuals for the RIO adapters, as well as the IO cards you plan to use, alot of good info just takes a bit to sort through it. Make a "quick reference guide" as you are reading through it all.

To add a little to the above, if you add the module to the I/O list ( not required in PLC5s)
the software will set the lengths for you and allow you to setup the configuration.

http://www.plctalk.net/qanda/showpost.php?p=27451&postcount=2

http://www.plctalk.net/qanda/showpost.php?p=137747&postcount=7