AB Quality ALLEN-BRADLEY 1791 User Manual page 38

Block transfer requires 1/4 rack.
MSB = Module Status Byte
MCB = Module Control Byte
When using block transfer, each analog block I/O module uses 2 words of
output image table memory and 2 words of input image table memory.
Each block occupies 1/4 rack of data table, with 4 blocks comprising 1
logical rack. Image table usage for one assigned rack number is shown in
Figure 3.3.
Figure 3.2
I/O Image Table for One Assigned Rack Number using Block Transfer
Chapter 3
Configuring Your Block I/O for PLC
Family Programmable Controllers
Input Image
17 10 7
MSB
0
Reserved
1
MSB
2
Reserved
3
4
5
6
7
Output Image
17 10 7
MCB
0
Reserved
1
MCB
2
Reserved
3
4
5
6
7
PLC
0
1/4 Rack
1/4 Rack
PLC
0
1/4 Rack
1/4 Rack
3-5