User manual

Chapter 5 PLC-5 BT Instructions

ADVANCED MICRO CONTROLS INC.

Continuous Parameter

The Continuous parameter controls how often the block transfer instruction is executed.

When the continuous parameter is set to "NO", the block transfer is executed only on a false to

true transition on the rung. This means that a non-continuous block transfer can occur at most

every other scan. When the continuous parameter is set to "YES", the block transfer will occur

when the BT instruction is first scanned and then every scan thereafter until an error in commu-

nication occurs.

Block Transfer Writes

to the 2762-17 module

MUST

have the Continuous

Parameter set to NO. Continuously writing Program Instructions to the module

may intefere with normal module operation.

Block Transfer Reads

to the 2762-

17 can have their Continuous Parameter set to "YES".

Enable (EN), Error (ER), and Done (DN) Bits

Used to signal the start and finish of a block transfer, the processor sets the EN bit to start

the transfer and after successfully completing the transfer the module sets the DN bit. If an

error occurs in the transfer, the module will set the ER bit instead of the DN bit.

The EN, ER, and DN bits are located in the first word of the Control File. The EN Bit is bit

15, the ER Bit is bit 12 and the DN Bit is bit 13.

The following warning is taken verbatim from Allen-Bradley's PLC-5 Family

Programmable Controllers Processor Manual, Publication 1785-6.8.2 - November, 1987 and

refers to the control bits of the BT instruction. These bits include the Enable, Error, and Done

bits.

IMPORTANT:

The processor executes block-transfer instructions asynchro-

nous to the program scan. The status of these bits could change at any point in

the program scan. When you test these bits (especially the done bit), test them

only once every ladder program scan. If necessary, set temporary storage bits

for the purpose of enabling subsequent rungs from them.

Also, your ladder program should condition the use of block transfer data on the

examination of the block-transfer error bit. An error may occur when the

processor is switched from run mode, or when processor communications are

interrupted."