Modbus slave count limitation

Question

I have two questions:

1. Why does Modbus allow only 247 slaves to be connected to the same network?

2. How it this limitation overcome by Modbus TCP/IP?

Answer 1

1) The address field in the Modbus-defined frame is only 8 bits wide. This would provide for 256 possible slave addresses. Address 0 is reserved as a "broadcast" address.

I can't find a specific reason, but there are presumably eight other "reserved" addresses that are not valid for a slave. 256 - 9 = 247.

2) Modbus TCP/IP is implemented atop standard IPv4 addressing, which is limited only to the size of the local subnet (e.g. 10.10.0.0). Each "gateway" on the network will have its own IP address, so there can be many more than 247 gateways.

In addition, each gateway can have multiple Modbus devices behind it. The number of devices behind such a gateway is limited by the same 8-bit address field, here called the "Unit Identifier".

Reference:

• Modbus Protocol Reference Guide
• "Modbus" (Wikipedia)