Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 62)

Choosing Module Features
Chapter 3
3Ć20
The highest number of words that you can transfer in one block transfer is
64. You must include two command words in each write block transfer
and two status words in each read block transfer in addition to your data
words. You can also transfer up to four initialization words (Figure 3.9).
Figure 2.9
Block Lengths for Read and Write Block Transfers
17 16 15 14 13 12 11 10 07 06 05 04 03 02 01 00
Command Word No. 1
Command Word No. 2
Initialize Data Word No. 1
Initialize Data Word No. 2
Initialize Data Word No. 3
Initialize Data Word No. 4
Initialization WRITE Block
17 16 15 14 13 12 11 10 07 06 05 04 03 02 01 00
Command Word No. 1
Command Word No. 2
WRITE Block
64 Words, max. Data
17 16 15 14 13 12 11 10 07 06 05 04 03 02 01 00
Status Word No. 1
Status Word No. 2
READ Block
64 Words, max. Data
The longest data string read from the ASCII device determines the block
length of the read block transfer instruction. In the PLC-2 family, the read
and write block lengths must be equal to ensure correct operation. For
PLC-3 processors, the block lengths can be different.
Determining Block Transfer
Length