Basler Electric BE1-851 Modbus Protocol - Manual (Page 11)

9289900992 Rev F BE1-851 Modbus
™ Protocol
1-3
Error Handling and Exception Responses
Any query received that contains a non-existent device address, a framing error, or CRC error is ignored.
No response is transmitted. Queries addressed to a BE1-851 with an unsupported function or illegal
values in the data block result in an error response message with an exception response code. The
exception response codes supported by the BE1-851 are provided in Table 1-2.
Table 1-2. Supported Exception Response Codes
Code Name Meaning
01 Illegal Function
The query Function/Subfunction Code is unsupported; query read
of more than 125 registers; query preset of more than 100 registers
02 Illegal Data Address
A register referenced in the data block does not support queried
read/write; query preset of a subset of a numerical register group.
03 Illegal Data Value
A preset register data block contains an incorrect number of bytes
or one or more data values out of range.
COMMUNICATIONS HARDWARE REQUIREMENTS
The BE1-851 RS-485 physical interface is three positions of a terminal strip with locations for
Send/Receive A (A), Send/Receive B (B), and Signal Ground (C). Refer to the BE1-851 Instruction
Manual (9289900990) for further details.
DETAILED MESSAGE QUERY AND RESPONSE
A detailed description of BE1-851 supported message queries and responses is provided in the following
paragraphs.
Read Holding Registers
Query
This query message requests a register or block of registers to be read. The data block contains the
starting register address and the quantity of registers to be read. A register address of N will read holding
register N+1. If the query is a broadcast (device address = 0), no response message is returned.
Device Address
Function Code = 03 (hex)
Starting Address Hi
Starting Address Lo
No. of Registers Hi
No. of Registers Lo
CRC Hi error check
CRC Lo error check
The number of registers cannot exceed 125 without causing an error response with the exception code
for an illegal function.
Response
The response message contains the data queried. The data block contains the block length in bytes
followed by the data (one Data Hi byte and one Data Lo byte) for each requested register.
Reading an unassigned holding register returns a value of zero.
Device Address
Function Code = 03 (hex)
Byte Count
Data Hi (For each requested register, there is one Data Hi and one Data Lo.)