Laurel Electronics LTSE6 - Manual - Manual (Page 17)

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9. MODBUS PROTOCOL TRANSMITTER COMMUNICATIONS
1. GENERAL
The Modbus capability conforms to the Modbus over Serial Line Specification & Implementation
guide, V1.0. Both the Modbus RTU and Modbus ASCII protocols are implemented:
Modbus RTU
Baud Rate........... 300, 600, 1200, 2400, 4800, 9600 or 19200
Data Format ....... 1 start bit, 8 data bits, 1 parity bit, 1 stop bit (11 bits total)
Parity.................. None, Odd, Even (if None, then 2 Stop bits for 11 total)
Address.............. 0 for broadcast, 1-247 for individual devices
Modbus ASCII
Baud Rate........... 300, 600, 1200, 2400, 4800, 9600 or 19200
Data Format ....... 1 Start bit, 7 Data bits, 1 Parity bit, 1 Stop bit (10 bits total)
Parity.................. None, Odd, Even (if None, then 2 Stop bits for 10 total)
Address.............. 0 for broadcast, 1-247 for individual devices
2. FRAMING
Modbus RTU: Message frames are separated by a silent interval of at least 3.5 character times. If a
silent interval of more than 1.5 character times occurs between two characters of the message
frame, the message frame is considered incomplete and is discarded. Frame Check = 16 bit CRC of
the complete message excluding CRC characters.
Modbus ASCII: The message begins immediately following a colon (:) and ends just before a
Carriage Return/ Line Feed (CRLF). All message characters are hexadecimal 0-9, A-F (ASCII
coded). The system allowable time interval between characters may be set to 1, 3, 5 or 10
seconds. Frame Check = 1 byte (2 hexadecimal characters) LRC of the message excluding the
initial colon (:) and trailing LRC and CRLF characters.
3. ELECTRICAL INTERFACE
Two-wire (plus common) half-duplex RS485 or RS232 signal levels are jumper selectable on the
transmitter main board. The RS485 selection provides a jumper selection for insertion of a line
termination resistor. Please see Section 14. In case of a long line (greater then 500 ft) to the first
device, a termination resistor should be selected for the first device. In case of a long line between
the first and last devices, a termination resistor should be selected for the first and last devices.
Never add termination resistors to more than two devices on the same line.