Beijer Electronics AN-BEI-P2-038 User Manual

Page 15

APPLICATION NOTE AN‐BEI‐P2‐038

Date: 15/02/12

AN‐BEI‐P2‐038 Modbus RTU Control and Register Mapping

Modbus Exception Response Telegrams
Under some circumstances, the drive may reply with an Exception Response (error) in response to a request telegram sent from
the network master, for example where the master tries to read a register which does not exist. Exception Responses which can
be generated by the drive are listed below :‐

Exception Code 1: Invalid Request
Returned under the following conditions

• Network Master sends an unsupported Modbus command (e.g. Read Coils).
• Run command issued to drive whilst the drive is not set for Modbus Control Mode (e.g. P1‐12 <>4).
• Run command issued to drives whilst the drive is not enabled (e.g. Digital Input 1 is open).
• Run command issued to drive whilst the drive is in a tripped condition.

Exception Code 2: Invalid Modbus register
Returned under the following conditions

• Network Master attempts to read or write a register that does not exist within the drive.

Exception Code 3: Register Value Out of Range
Returned under the following conditions

• Network Master attempts to write a holding register with a value outside the range of the register

Exception Code 6: Drive Busy
Returned under the following conditions

• Drive busy due to internal data transfer. The Network Master should re‐send the message after a delay

Dataflow example:
Read data from register 6:

Request: [01] [03] [00] [05]

[00] [01] [94] [0B]

(Drive Addr) (Command) (Reg start addr) (No. of Registers) (Checksum)

Reply: [01] [03] [02]

[00] [00] [B8] [44]

(Drive Addr) (Command) (No of data bytes) (Data) (Checksum)

Note: The actual start address of register 6 is 5. All data in [ ] is in 8bits Hex format.

Write start command to the register 1 (suppose P1‐12 = 4, P1‐13 =1 and digital input 1 is closed):

Request:

[01]

[06]

[00] [00]

[00] [01]

[48] [0A]

(Drive Addr)

(Command)

(Reg addr)

(Data value)

(Checksum)

Reply: