B&B Electronics MES1A - Manual (Page 59)

Modbus Basics
Manual Documentation Number: MES1A/MES1B-2106m PN7138-rev001
B&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 – www.bb-elec.com
B&B Electronics Ltd – Westlink Commercial Park – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445 – www.bb-europe.com
49
Another aspect of Modbus/TCP that must be considered relates to the Unit
ID field. Modbus/RTU incorporates the slave address field but Modbus/TCP
replaces the slave address field with a Unit ID field. TCP/IP uses IP
addresses to find specific nodes on the network. By using the IP address to
specify the node, and the Unit ID to specify a Modbus/RTU device attached
to that node, multiple slaves can be located at a single IP address.
Unfortunately, some manufacturers of early Modbus/TCP systems assumed
only one slave per IP address and always used a Unit ID of 00. If this early
equipment is used, only one Modbus/RTU slave can be connected to an
MES1A/B. (The MES1B is capable of RS-485 multidrop, allowing multiple
slaves.)
Hints and Tips
A few simple suggestions that may assist you if your system is experiencing
problems include:
• Slow down the polling rate.
• Try the Disable Pipeline option in the MES1A/B configuration menu.
• Make sure your Modbus/TCP equipment uses sequence and Unit ID
numbers.