GE Industrial Solutions A Series Lighting Control Panelboards Modbus Register Map - Manual (Page 7)

A Series ® Lighting Controller Modbus Register Map
Introduction
1
Introduction
The Lighting Controller can be configured as a Modbus
Slave device using the RTU (Remote Terminal Unit)
Modbus Master-Slave protocol. The Lighting Controller is
always the slave and cannot initiate communications.
MODBUS devices communicate using a master-slave
technique in which only one device (the master) can
initiate transactions (called queries). The other devices
(slaves) respond by supplying the requested data to the
master, or by taking the action requested in the query. A
slave is any peripheral device (I/O transducer, valve,
network drive, or other measuring device), which
processes information and sends its output to the master
using MODBUS. Masters can address individual slaves, or
can initiate a broadcast message to all slaves. Slaves return
a response to all queries addressed to them individually,
but do not respond to broadcast queries
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.