Specifications

VersaPro Temperature Controller Page 49

The Function Code field tells the addressed slave what function to perform. MODBUS function codes are

specifically designed for interacting with a PLC on the MODBUS industrial communications system. Command

codes were established to manipulate PLC registers and coils. As far as the VersaPro is concerned, they are

all just memory locations, but the response to each command is consistent with Modbus specifications.

The high order bit in this field is set by the slave device to indicate an exception condition in the response

message. If no exceptions exist, the high-order bit is maintained as zero in the response message.

Data Field

The data field contains information needed by the slave to perform the specific function or it contains data

collected by the slave in response to a query. This information may be values, address references, or limits.

For example, the function code tells the slave to read a holding register, and the data field is needed to

indicate which register to start at and how many to read.

Error Check Field (CRC)

This field allows the master and slave devices to check a message for errors in transmission. Sometimes,

because of electrical noise or other interference, a message may be changed slightly while it is on its way from

one device to another. The error checking assures that the slave or master does not react to messages that

have changed during transmission. This increases the safety and the efficiency of the MODBUS system.

The error check field uses a CRC-16 check in the RTU mode.

The following is an example of a function 03 call for timer setpoint value (TSETPT) at memory location 03. The

value returned by the instrument is the hex value 1E (30 seconds).

Transmit from Host or Master

Address Cmd Reg HI Reg LO Count HI Count LO CRC HI CRC LO

01 03 00 03 00 01 74 0A

Response from Versapro

Address Cmd Byte

Count HI

Byte Count LO Data HI Data LO CRC

CRC Lo

01 03 00 02 00 1E 38 4C

Note that all the values are interpreted as hexadecimal values. The CRC calculation is based on the A001

polynomial for RTU Modbus. The function 04 command structure is similar to the 03 structure.

The following is an example of a function 06 call to change the remote setpoint (RSETPT) to 200 (2.00%). The

response from the instrument confirms the new value as being set.

Transmit from Host or Master

Address Cmd Reg HI Reg LO Data HI Data LO CRC HI CRC LO

01 06 00 01 00 C8 D9 9C

Response from Versapro

Address Cmd Reg HI Reg LO Data HI Data LO CRC HI CRC LO

01 06 00 01 00 C8 D9 9C