GE Industrial Solutions Record Plus FG600 SMR2 Modbus - Manual (Page 4)

SMR2 Modbus User Manual
2
Introduction
The SMR2 trip unit has an optional communications
module that facilitates monitoring of the trip unit via a
master Modbus device. Monitoring features include
breaker settings and the breaker status.
Communication with the SMR2 trip unit requires a
FAMECM module powered externally that provides
isolation and protection.
The Modbus communication protocol is a single
master multi-slave protocol that is hardware
independent and it interfaces with a 2-wire RS485
interface. A single master device initiates and controls
all communication with the other slave devices on the
network. A computer with a serial port and an external
RS-232/RS-485 converter can be used as the master-
monitoring device. The SMR II trip unit is always the
slave and cannot initiate communication. The
maximum number of devices on a Modbus network is
32, i.e. only 31 slave devices can be connected to a
master.
MODBUS devices usually include a Register Map.
MODBUS functions operate on Register map registers
to monitor, configure, and control module I/O. You
should refer to the Register map for your device to gain
a better understanding of its operation.
The transmission mode defines the bit contents of the
message bytes transmitted along the network, and how
the message information is to be packed into the
message stream and decoded.
Standard MODBUS networks employ one of two types
of transmission modes:
i) ASCII Mode ii) RTU Mode
The mode of transmission is usually selected along
with other serial port communication parameters
(baud rate, parity, etc.) as part of the device
configuration.
RTU Transmission Mode
In RTU (Remote Terminal Unit) Mode, each 8-bit
message byte contains two 4-bit Hexadecimal
characters, and the message is transmitted in a
continuous stream. The greater effective character
density increases throughput over ASCII mode at the
same baud rate.
Message frame
A message frame is used to mark the beginning and
ending point of a message allowing the receiving device
to determine which device is being addressed and to
know when the message is completed. It also allows
partial messages to be detected and errors flagged as a
result.
A MODBUS message is placed in a message frame by
the transmitting device. Each word of this message
(including the frame) is also placed in a data frame
that appends a start bit, stop bit, and parity bit.
In ASCII mode, the word size is 7 bits, while in RTU
mode; the word size is 8 bits. Thus, every 8 bits of an
RTU message is effectively 11 bits when accounting for
the start, stop, and parity bits of the data frame.
RTU Mode Message Frames
RTU mode messages start with a silent interval of at
least 3.5 character times. Implemented as a multiple of
character times at the baud rate being used on the
network. The first field transmitted is the device
address. The allowable characters transmitted for all
fields are hexadecimal values 0-9, A-F. A networked
device continuously monitors the network, including
the silent intervals, and when the first field is received
(the address) after a silent interval of at least 3.5
character times, the device decodes it to determine if it
is the addressed device. Following the last character
transmitted, a similar silent interval of 3.5 character
times marks the end of the message and a new message
can begin after this interval.
The entire message must be transmitted as a
continuous stream. If a silent interval of more than 1.5
character times occurs before completion of the frame
(not a continuous stream), the receiving device flushes
the incomplete message and assumes the next byte will
be the address field of a new message.
In similar fashion, if a new message begins earlier than
3.5 character times following a
previous message, the receiving device assumes it is a
continuation of the previous message. This will
generate an error, as the value in the final CRC field
will not be valid for the combined messages.
Message Format
The Modbus RTU Protocol is strictly based upon a
transaction scheme where a master device generates a
query and a slave device replies with a response. Each
query and response message transaction consists of the
following four parts