Issue
What is the maximum number of registers that can be read/written using the multiple read/write function codes in Modbus
Product Line
EcoStruxure Building Operation
Environment
- Building Operation Automation Server
- Modbus Register Groups
Cause
When deciding how to group registers using a Modbus Register Group, what are the maximum number of registers that can be grouped.
Resolution
There is a limit to the number of packets that can be polled in a single packet, however, if this number is exceeded then two or more polls will be made (blocks). The nominal Read multiple registers maximum is 125 16 bit registers (250 8-bit bytes, so the register count will vary for 32 and 64bit polls). The Register Group Properties Tab shows the number of blocks being polled.
The Modbus register grouping function has a direct effect on system performance. A poorly grouped device can easily increase the overall poll time as points, or blocks, in each Modbus network are polled sequentially. See the section "Batch Read Function" in the EcoStruxure Building Operation Technical Reference Guide for full details.
Note: Multiple group writes are not possible and are not controlled via Modbus Register Groups.
