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GEK-100682D
7 MODBUS PROTOCOL
This chapter describes the functional specifications of the Modicon Modbus protocol for interface module for
the Microprocessor Distribution Protection (MDP Relay) Digital Time Overcurrent Relaying System. The speci-
fications cover Modicon Modbus application messages supported by the MDP interface software module. The
objective is to move the MDP relay from communicating through a concentrator to direct connection to Mod-
bus. The MDP direct interface should provide all the same functions as were implemented using the concen-
trator interface, with the addition of breaker control.
Additional information on the Modbus protocol can be found at www.modbus.org.
The Modicon Modbus interface module allows the MDP to communicate with the Power Leader Distribution
Software (PLDS) running on a host computer via Modbus. The Modbus interface card is built into the MDP
relay unit and communicates with the relay via a TTL level serial asynchronous protocol. The Modbus interface
card does not provide any LED display or key switches as provided for other devices. The MDP relay provides
the user interface. The Modbus device address for the MDP unit is configurable from the switches provided on
the MDP relay itself; the specified address is read by the Modbus interface module using the serial protocol.
The baud rate for serial communication is internally fixed at 2400 baud and is not configurable by the user. The
Modbus card can be configured externally for the following baud rates: 1200, 2400, 9600, and 19200 baud.
The Modbus interface module provides the protocol conversions required to allow full operability on a Power
Leader network.
The Modus interface module provides the commands described below. Although some of the standard com-
mands are specified in the Modicon Modbus reference guide PI-MBUS-300 REV.E (see www.modbus.org for
additional information), they are described here for the sake of completeness.
a) READ HOLDING REGISTERS, FUNCTION CODE 03
Modbus description: Reads the binary contents of the MDP holding registers. Broadcast is not supported.
The following MDP registers are available to read: Unit ID/Modbus address, Setpoint parameters, COC soft-
ware version, Product revision, and MDP software revision.
•
QUERY: The query message specifies the starting register and quantity of registers to be read. Registers
are addressed starting at zero; i.e., registers 1 to 16 are addressed as 0 to 15.
•
RESPONSE: The register data in the response message are packed as two bytes per register, with the
binary contents right justified within each byte. For each register, the first byte contains the high order bits
and the second contains the low order bits.
Data is scanned in the slave at the rate of 125 register per scan each register contains two bytes.
GE Power Management
Courtesy of NationalSwitchgear.com
MDP Digital Time Overcurrent Relay
7 MODBUS PROTOCOL
7.1 DESCRIPTION
7.2 OVERVIEW
7.3 MODBUS COMMANDS
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