GE Industrial Solutions Record Plus FG600 SMR2 Modbus - Manual (Page 5)

SMR2 Modbus User Manual
MODBUS ADDRESSES
The master device addresses a specific slave device by
placing the 8-bit slave address in the address field of the
message (RTU Mode). The address field of the
message frame contains two characters (in ASCII
mode), or 8 binary bits (in RTU Mode). Valid
addresses are from 1-247.
When the slave responds, it places its own address in
this field of its response to let the master know which
slave is responding.
MODBUS FUNCTIONS
The function code field of the message frame will
contain two characters (in ASCII mode), or 8 binary
bits (in RTU Mode) that tell the slave what kind of
action to take. Valid function codes are from 1-255, but
not all codes will apply to a module and some codes are
reserved for future use.
MODBUS DATA FIELD
The data field provides the slave with any additional
information required by the slave to complete the
action specified by the function code. The data is
formed from a multiple of character bytes (a pair of
ASCII characters in ASCII Mode), or a multiple of two
hex digits in RTU mode, in range 00H-FFH. The data
field typically includes register addresses; count values,
and written data. If no error occurs, the data field of a
response from a slave will return the requested data. If
an error occurs, the data field returns an exception
code that the master's application software can use to
determine the next action to take.
CRC Error Checking
RTU Mode message frames include an error checking
method that is based on a Cyclical Redundancy Check
(CRC). The error-checking field of a message frame
contains a 16-bit value (two 8-bit bytes) that contains
the result of a Cyclical Redundancy Check (CRC)
calculation performed on the message contents.
The checksum field lets the receiving device determine
if a packet is corrupted with transmission errors. In
Modbus RTU mode, a 16-bit Cyclic Redundancy Check
(CRC-16) is used. The sending device calculates a 16-
bit value, based on every byte in the packet, using the
CRC-16 algorithm. The calculated value is inserted in
the error check field.
The receiving device performs the calculation, without
the error check field; on the entire packet it receives.
The resulting value is compared to the error check
field.
Transmission errors occur when the calculated
checksum is not equal to the Checksum stored in the
incoming packet. The receiving device ignores a bad
packet.
Electrical Interface
The electrical interface is 2-wire RS485 where data flow
is bi-directional and half duplex. RS-485 lines from the
master device should be connected to a FAMECM
module, which is powered externally using a 19-29 VDC
power supply. Refer Figure 1 and 2.
The following wire types are recommended for network
wiring: Belden 3074F – Data Tray 600 V industrial
twinax 18 AWG (7X26); or Belden 9841 – 300 V
communication cable 24 AWG (7X32); or Alpha 6412 –
300 V Communication cable 24 AWG (7x32). Shielded
wire should always be used to minimize noise. Polarity
is important in RS485 communications and each '+'
terminal of every device must be connected together for
the system to communicate.
Figure1. Connection between ETU and FAMECM
module
2