Laurel Electronics LTM DPM SERIES ANALOG INPUT, SERIAL DATA OUTPUT - Manual (Page 26)

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19. MODBUS PROTOCOL TRANSMITTER COMMUNICATIONS
1.0 GENERAL
The Modbus capability conforms to the Modbus over Serial Line Specification & Implemen-
tation 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 meters
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 meters
2.0 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.
3.0 ELECTRICAL INTERFACE
RS232, two-wire half-duplex RS485, or four-wire full-duplex RS485 signal levels are selectable
via jumpers on the transmitter main board and a the connector. Please see Section 13. The
RS485 selection provides a jumper selection for insertion of a line termination resistor. 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.