Rockwell Automation 1771-DB Basic Module User Manual - Series A - Manual (Page 139)

Programming
Chapter 6
6–7
Rung 1 — Rung one is true only at power-up. It uses status word 3, bit 3
(the AC power loss bit of the PLC-3) to zero the control file of both the
BTR and BTW.
Rungs 2 and 3 — During normal program execution the BTW and BTR
instructions are alternately executed. The done bits of each instruction
enable the next block-transfer instruction. The BTR and BTW control files
must be different for the next block-transfer to occur.
The equal instruction is used at power-up. At power-up the BTR and BTW
control files both equal zero. At power-up the BTW enables and
block-transfers begin.
You can use the following ladder logic program with PLC-5 Family
processors. This program assumes that your application requires a single
BTR and BTW to pass data between the processor and the BASIC Module
(i.e. transfer of 64 words or less). If the transferred data exceeds 64 words
you must program multiple file to file moves to move different data sets to
and from the block-transfer files.
The PLC-5 block-transfer program is an alternating read/write program
(i.e. at the completion of the BTW the BTR enables. At the completion of
the BTR the BTW enables). The processor checks data validity before
accepting read data and sets one enable bit at a time. Figure 6.6 is a generic
sample block-transfer program for a PLC-5 family processor. Figure 6.7 is
an actual sample program.
6.4.1
Rung Description for Sample
PLC-3 Family Ladder Logic –
Single Data Set
6.5
PLC-5 Family Processors