Basler Electric DECS-400 - Manual (Page 215)

9369700990 Rev R 203
Modbus
™ Communication
This chapter describes the Modbus communication protocol employed by the DECS-400, and how to
exchange information with the DECS-400 over a Modbus network. The DECS-400 communicates by
emulating a subset of the Modicon 984 Programmable Controller.
DECS-400 Modbus
™ Protocol
Modbus communication uses a master-slave technique in which only the master can initiate a transaction.
This transaction is called a query. When appropriate, a slave (DECS-400) responds to the query. When a
Modbus master communicates with a slave, information is provided or requested by the master.
Information residing in the DECS-400 is grouped characteristically in categories. The following information
categories are maintained by the DECS-400:
C1: Version data registers
C2: Metering registers, group 1
C3: System configuration registers
C4: Operating mode parameter registers
C5: Setpoint parameter registers
C6: Startup parameter registers
C7: Limiter parameter registers
C8: Gain registers
C9: Protection function parameter registers
C10: Exciter diode monitor parameter registers
C11: Relay parameter registers
C12: ASCII and Modbus communication parameter registers
C13: Metering registers, group 2
C14: Power system stabilizer parameter registers
All supported data can be read or written as specified in the register table. Abbreviations are used in the
register table to indicate the register access type. Register access types are read/write (RW) and read
only (R).
All categories except metering (C2 and C13) and version data (C1) can generally be written via a Modbus
message as well as read. Categories C11, C12, and C15 are strictly read-only.
When a slave receives a query, the slave responds by either supplying the requested data to the master
or performing the requested action. A slave device never initiates communications on the Modbus, and
will always generate a response to the query unless certain error conditions occur. The DECS-400 is
designed to communicate on the Modbus only as a slave device.
A master can only query slaves individually. If a query requests actions unable to be performed by the
slave, the slave response message contains an exception response code defining the error detected.
Message Structure
Master initiated queries and DECS-400 (slave) responses share the same message structure. Each
message is comprised of four message fields. They are:
•
Device Address (1 byte)
•
Function Code (1 byte)
•
Data Block (n bytes)
•
Error Check field (2 bytes)
Device Address Field
The device address field contains the unique Modbus address of the slave being queried. The addressed
slave repeats the address in the device address field of the response message. This field is 1 byte.
DECS-400 Modbus™ Communication