B&B Electronics MES1A, MES1B Allow Modbus Broadcasts , Use MB/TCP 00BH/00AH Exception Responses

Configuring the MES1A/B

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 30

• If any Modbus TCP masters on your system use 0 for all Unit ID fields,

select a slave address that matches the Modbus/RTU slave attached to

your MES1A/B. You will not be able to connect multiple slaves to the

RS-485 multidrop bus.

Allow Modbus Broadcasts

If you configured the Slave Address for any number between 1 and 255 the

Allow Modbus Broadcasts does not appear in the menu.

When a Modbus/RTU slave receives a slave address of 0 it interprets this as a

broadcast message. All slaves accept the message. Setting the Allow

Modbus Broadcasts (1=Yes, 2=No)to No reassigns a Unit ID of 0 from

a Modbus TCP master as a slave address 1. This ensures that a Unit ID is not

interpreted as a broadcast message.

Use MB/TCP 00BH/00AH Exception Responses

In Modbus/RTU, when a master initiates a query it expects to receive a

response. If it does not receive one within an acceptable length of time it

interprets the silence as an indication that a communications error has

occurred. Because delays in TCP/IP networks are not predictable, problems

can occur. To address this, Modbus/TCP defines two exception codes that

can be enabled to be sent in the event of a communications failure.

If Use MB/TCP 00BH/00AH Exception Responses (1=No 2=Yes)is

disabled (1=No), the MES1A/B operates like a Modbus/RTU slave. It

answers timeouts, un-configured slave addresses and CRC errors with

silence.

If the Use MB/TCP 00BH/00AH Exception Responses (1=No

2=Yes)is enabled (2 = Yes), the MES1A/B will return the following codes

under any of the conditions described:

0A (hex) - Path Unavailable

• If it receives a query from a Modbus master in which the Unit ID

specifies a Slave Address not listed in the Unit ID to IP Mapping Table

• If the TCP socket fails to open.