User manual

Chapter 5 PLC-5 BT Instructions
5-2 ADVANCED MICRO CONTROLS INC.
File Length (continued)
Block Transfer Writes
When programming a BTW instruction, you must specify the exact number of words that
contain your programming instructions. You cannot use a file length of zero with a BTW
instruction unless you are actually transmitting sixty-four words. If you do so, the module will
issue an error message to the processor.
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 a 1700 module
must
have the Continuous Parameter
set to “NO”. Continuously writing Program Instructions to the module may
interfere with normal operation.
Block Transfer Reads
to a 1700 module 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 Programma-
ble 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 asynchronous
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.”