Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 69)

Operating Your PLC-5 Backup System
Chapter 5
5-5
Important: When programming block transfer instructions, you must specify
the LENGTH field (Figure 5.3) as 0, since the 1785-BCM module determines
that size in accordance with its status (primary or secondary) and type of
instruction (BTW or BTR).
Figure 5.3
Block Transfer Instructions
EN
BTR
Rack
Group
Module
Control Block
Data file
0
0
0
N10:100
DN
Length
Continuous
BLOCK TRNSFR READ
N10:110
0
N
ER
EN
BTW
Rack
Group
Module
Control Block
Data file
0
0
0
N10:0
DN
Length
Continuous
BLOCK TRNSFR WRITE
N10:10
0
N
ER
Specify field as 0
11055I
Block Transfer Read (BTR) Instruction
The local 1785-BCM module provides information to the ladder logic to help
coordinate the orderly exchange of information from the primary processor to
the secondary processor. The local processor obtains this information by
executing a BTR instruction. As shown in Table 5.A, the first two words of the
block transfer read (BTR) instruction contain:
status information
the current block identification number (block ID)
The BTR in the primary processor transfers only these two words; the BTR in
the secondary processor transfers 64 words — the system status and block ID
words, plus 62 words of data. Figure 5.4 shows the words of the BTR
instruction.