B&B Electronics MES1A - Allow Modbus Broadcasts; Use MB;TCP 00 BH;00 AH Exception Responses

Configuring the MES1A/B
Manual Documentation Number: MES1A/MES1B-2106m PN7138-rev001
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• 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.
1 = No
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.
2 = Yes
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.