Xylem H-3531 FlashLite - Manual (Page 73)

Appendix B
Modbus Protocol
H-3531
Modbus Protocol B-1
B.1 Modbus Interface
Modbus is an industry standard field bus for interconnecting Programmable Logic Controllers
(PLCs), intelligent sensors and other devices. The H-3531 FlashLite™ is a Modbus slave and has
a serial RS-485 port for connecting to a Modbus compatible host device. See Chapter 2 for the
proper connections. Continuous +12Volt power must be supplied to the H-3531 FlashLite™.
B.2 Modbus RTU Transmission
The H-3531 FlashLite™ supports RTU (Remote Terminal Unit) transmission mode. The host
must support RTU mode to communicate with the H-3531 FlashLite™. The RTU framing format
for each byte is fixed at 11-bits and is as follows: 1 start bit, 8 data bits, 1 parity bit, and 1 stop bit.
The H-3531 FlashLite™ defaults to even parity but the user can change this setting. In RTU
transmission mode all request and response bytes are sent as two 4-bit hexadecimal characters.
RTU transmission mode also defines certain message frame timing constraints, this will not be
discussed in this document but the user can find the RTU mode transmission specifications online
at
www.modbus.org.
B.3 Modbus Function codes
The H-3531 FlashLite™ supports two Modbus function codes as shown in the table below.
Table 1 - Supported Function Codes
Function Code Description # of Registers to R/W
03
16 Read Holding Registers
Write Multiple Registers 33
33