# Modbus RTU Protocol

## Communications format

bit order: lsb first

byte order: big endian

Data bits:8 

Parity: None 

Stop bits: 1

Floating point number follows IEEE754-1985. The float32 below stands for 32bits Single precision floating point number.

## Terminology

‌‌Register: Length of 2 Byte or 16bit which is unit length of memory in Modbus RTU.

‌Byte: 8 bit length

## Definition and function of memory

Modbus memory is divided into two categories, read-only and read-write.

Read-only type is mainly for flow meter status related memory, such as instantaneous flow rate. This is also the most commonly used memory. Detail information in following page:

[Read-only memory read instruction(04) and memory lists](/qr/fp-as510-paddle-wheel-flowmeter-manual-en-v2/communication/modbus-rtu-protocol/read-only-memory-read-instruction-04-and-memory-lists.md)

Read-write memory is mainly to set-up parameters such as pipe diameter, and to reset variables, such as set zero on the cumulative flow. Read-write memory is only readable by using instruction 03. You can only change one parameter at a time with the write operation. If the length of the target register is 1, please use instruction 06. If the length is 2 registers, please use instruction 30. Please see read-write memory detail page below:

[Read-write memory list](/qr/fp-as510-paddle-wheel-flowmeter-manual-en-v2/communication/modbus-rtu-protocol/read-write-memory-list.md)

Read-write memory instruction detail information as below:

[Read-write memory read instruction(03)](/qr/fp-as510-paddle-wheel-flowmeter-manual-en-v2/communication/modbus-rtu-protocol/read-write-memory-read-instruction-03.md)

[Read-write memory single Register write instruction(06)](/qr/fp-as510-paddle-wheel-flowmeter-manual-en-v2/communication/modbus-rtu-protocol/read-write-memory-single-register-write-instruction-06.md)

[Read-write memory double Register write instruction(30)](/qr/fp-as510-paddle-wheel-flowmeter-manual-en-v2/communication/modbus-rtu-protocol/read-write-memory-double-register-write-instruction-30.md)


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