PLC-5 Instruction BTW/BTR


Thread Starter


Just need to understand...

What these instructions do, and the interpolation with the control bits assigned to them. I have looked at the help on the programming
software, but still am confused as to what is going on. Anybody able to help would be much apprec.
You have to use BTR/BTW to exchange informations with analog devices, panel view...
For example with an analog input module (1771 IFE):
- with the BTW instruction, you send values to configure the module (scale for each channel...).
- with the BTR instruction, you read the analog values for each channel from the module.
You can use the status of BTR/BTW (.en,.dn,.err) in your ladder program.

Theo Baarslag

This instruction makes the PLC processor instruct the communications processor to either write (BTW) or read (BTR) information to/from a selected address on for instance the remote I/O channel. For this it needs the information itselves (writing)/an address to put it
(reading), and a control register, as well as what you are writing to, its address, etc.

Typically you would use block transfers for communicating with a PanelView, but you can also use this instruction if you have a module in the rack that takes more then just I/O allocation. This is the case for the configuration of analog
modules, counter/frequency modules, etc., as well as reading from /writing to Frequency drives, soft starters, etc.

Let me know which one you use, and I can help in more detail.


Theo Baarslag
PLC-/CIMspecialist for
IAMS Europe bv,
Coevorden The Netherlands
email: [email protected]
Giving you the short version. Block transfer reads and writes are for communications to "smart Cards" ... for instance analog cards in AB PLC 5. You BTW the configuration to the card and then BTW read the values coming into the card which are processed by the analof card and then are "read" into the PLC.

In an analog output card you read and write to the card all the time if you like....IE you write out the value to the card and then it processes this digital to analog conversion and scaling etc. and then passes it out its 4-20 ma (for example) cable. You read in statuses of that transaction. this is one example.

I beleive that the card communicatis on the PLC backplane in the place ot the 16 IO bits in and out back and forth to the card......

As the documentation says, the rest are set up and scaling etc........

Hard to detail in 3 paragraphs or less..

DAVCO Automation
"The Developing Automation Value Company"