PLC-5 Block Transfer--help required from experts

I have a PLC-5 running system with one local and 3 remote chassis. There are few digital Input(1771-ICD) and Digital output(1771-OW16) modules in Remote Racks. PLC-5 is accessing Analog inputs in remote racks by BTR and BTW block transfer instruction. But how PLC-5 is acessing these digital inputs and outputs with out any BTR or BTW instruction in the entire logic and also no configuration is done in the chassis and IO configuration. Yes the scanned list is defined but is it enough to acess block transfer digital modules (without any BTR or BTW instruction)

Digital I/O does not need block transfers.

Hi, salman111a

Each logical Rack of I/O is assigned 8 16-bit input words, and 8 16-bit output words in the PLC I/O image tables for the digital I/O data.

Block Transfers are a method of transferring more than 1 word (each-way) of data to a single slot - as is required for analog and other modules.

Hope this helps yourt understanding.

Greetings salman111a ...



my distinguished colleague daba has already given you the correct answer - but I’ll try to add just a little more detail ...



with the PLC-5 platform, it is NOT necessary to “fill out” the I/O Configuration table to make DIGITAL modules work ... specifically, most digital I/O modules do NOT need to be listed in order to work properly ...

note that this is DIFFERENT from the SLC-500 and the ControlLogix platforms where ALL of the modules (whether digital or analog) MUST be listed in order to work correctly ... (perhaps that is part of your problem in understanding?) ... over the years, I have had many students who understand the SLC-500 platform quite well - and then become surprised and confused to find that the PLC-5 platform does NOT require the I/O modules to be listed ... many have asked: "Where are all of the modules?" when they first access the I/O Configuration table ...

Click to expand...

going further: digital modules do NOT require Block Transfer instructions in order to work correctly ...



on the other hand, the analog I/O modules DO require Block Transfer instructions in order to work correctly ...



but ...



even with the analog I/O modules it is NOT necessary to list the modules in the I/O Configuration table ... in other words, as long as the proper “configuration” values are somehow stored in the processor’s data table (example: at N7:__, etc.) then the module CAN work reliably - just as long as the Block Transfer instructions are set up to properly carry the information to and from the analog modules ...



here’s another way of looking at the system that might be helpful ...



information TO and FROM the digital I/O modules appears directly on the processor’s Input and Output data tables ... specifically, this type of digital information will have addresses which start with I: or O: ...



on the other hand, information TO and FROM the analog I/O modules will NOT appear on the processor’s Input and Output data tables ... specifically this type of analog information will appear in the Integer data tables and will have addresses which start with “N” ...



the important point is this:



I: and O: addresses are AUTOMATICALLY moved into and out of the Input and Output data tables during the processor’s normal scan sequence ... on the other hand, the N-type addresses must be moved into and out of the Integer data tables by Block Transfer instructions ... specifically, these N-type addresses will NOT be automatically moved by the processor ...



I hope this helps you understand what is going on ...

Thank you so much
I have sucessfully added the Digital Input and Output modules with out any configuration in the PLC.
Incase of Analog i have sucessfully made BTR /BTW words to configure(scale) module and have retrieved data from Analog inputs sucessfully.

Thanks again for your response