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8.2 MODBUS PROTOCOL
8.2 MODBUS PROTOCOL
This section dedicated to discussion of details of the Modbus protocol. As noted above, specifics of DNP are
best obtained directly from the DNP Users Group at www.dnp.org. Along with the Device Profile Document,
the DNP specification provides sufficient information for a user to develop an interface should DNP wish to be
used for communications with the relay.
The GE Power Management 745 Transformer Management Relay implements a subset of the AEG Modicon
Modbus serial communication standard. Many devices support this protocol directly with a suitable interface
card, allowing direct connection of relays. The Modbus protocol is hardware-independent; that is, the physical
layer can be any of a variety of standard hardware configurations. This includes RS232, RS422, RS485, fibre
optics, etc. The 745 includes a front panel RS232 port and two rear terminal RS485 ports, one of which can be
configured as a four-wire RS422 port. Modbus is a single master / multiple slave protocol suitable for a multi-
drop configuration as provided by RS485/RS422 hardware. In this configuration up to 32 slaves can be daisy-
chained together on a single communication channel.
The GE Power Management 745 is always a Modbus slave. It cannot be programmed as a Modbus master.
The Modbus protocol exists in two versions: Remote Terminal Unit (RTU, binary) and ASCII. Only the RTU ver-
sion is supported by the 745. Monitoring, programming and control functions are possible using read and write
register commands.
Additional information on the Modbus protocol can be found on the Modbus website at www.modbus.org.
The hardware or electrical interface is any of the following:
•
two-wire RS485 for the rear terminal COM1 and COM2 terminals
•
four-wire RS422 for the rear terminal COM1 terminals
•
RS232 for the front panel connector
In a two-wire RS485 link, data flow is bi-directional. The four-wire RS422 port uses the RS485 terminal for
receive lines, and two other terminals for transmit lines. In the front panel RS232 link there are separate lines
for transmission and reception as well as a signal ground wire. In all configurations data flow is half duplex.
That is, data is never transmitted and received at the same time.
RS485 and RS422 lines should be connected in a daisy chain configuration (avoid star connections) with ter-
minating resistors and capacitors installed at each end of the link, i.e. at the master end and at the slave far-
thest from the master. The value of the terminating resistors should be equal to the characteristic impedance of
8
the line. This is approximately 120 Ω for standard 24 AWG twisted pair wire. The value of the capacitors should
be 1 nF. Shielded wire should always be used to minimize noise. Polarity is important in RS485 communica-
tions. The '+' terminal of every device must be connected together for the system to operate.
One data frame of an asynchronous transmission to or from a GE Power Management 745 consists of 1 start
bit, 8 data bits, and 1 stop bit. This produces a 10 bit data frame. The 745 can be configured to include an addi-
tional even or odd parity bit if required, producing an 11 bit data frame.
All ports of the GE Power Management 745 support operation at 300, 1200, 2400, 9600, and 19200 baud.
8-2
8.2.2 GE POWER MANAGEMENT MODBUS PROTOCOL
745 Transformer Management Relay
8 COMMUNICATIONS
8.2.1 DESCRIPTION
8.2.3 ELECTRICAL INTERFACE
8.2.4 DATA FRAME FORMAT AND RATE
GE Power Management
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