Modbus Basics

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.

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 815-433-5104www.bb-elec.com

B&B Electronics Ltd – Westlink Commercial Park – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445www.bb-europe.com

49

Page 59
Image 59
B&B Electronics MES1A, MES1B manual Hints and Tips