# Modbus RTU Protocol

## 通訊格式

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.

## 名詞定義

Register: 2 Byte 或 16bit 長，是 Modbus RTU 的記憶體通訊的單位長度。

Byte: 8 bit長

## 記憶體定義及功能

modbus 可操作的記憶體分為兩大類，唯讀型及讀寫型。

唯讀型主要是流量計狀態相關的記憶體，例如即時流量。這部分也是最常使用的記憶體。唯讀型的記憶體只能使用指 04 來讀取。相關頁面如下：

{% content-ref url="modbus-rtu-protocol/read-only-memory-read-instruction-04-and-memory-lists" %}
[read-only-memory-read-instruction-04-and-memory-lists](https://docs.lorric.com/qr/fp-as510-paddle-wheel-flowmeter-manual-chinese-v1/communication/modbus-rtu-protocol/read-only-memory-read-instruction-04-and-memory-lists)
{% endcontent-ref %}

讀寫型主要是流量計設定參數記憶體，例如管徑；還有重設變數，例如累計流量歸零。讀寫型的記憶體只能使用指 03 來讀取。寫入操作一次只能改變一個參數，如果目標參數的記憶體長度是 1個 register，使用指令 06，如果目標參數的記憶體長度是 2 register，使用 30。讀寫型記憶體列表頁面如下：

{% content-ref url="modbus-rtu-protocol/read-write-memory-list" %}
[read-write-memory-list](https://docs.lorric.com/qr/fp-as510-paddle-wheel-flowmeter-manual-chinese-v1/communication/modbus-rtu-protocol/read-write-memory-list)
{% endcontent-ref %}

讀寫型記憶體指令說明頁面如下：

{% content-ref url="modbus-rtu-protocol/read-write-memory-read-instruction-03" %}
[read-write-memory-read-instruction-03](https://docs.lorric.com/qr/fp-as510-paddle-wheel-flowmeter-manual-chinese-v1/communication/modbus-rtu-protocol/read-write-memory-read-instruction-03)
{% endcontent-ref %}

{% content-ref url="modbus-rtu-protocol/read-write-memory-single-register-write-instruction-06" %}
[read-write-memory-single-register-write-instruction-06](https://docs.lorric.com/qr/fp-as510-paddle-wheel-flowmeter-manual-chinese-v1/communication/modbus-rtu-protocol/read-write-memory-single-register-write-instruction-06)
{% endcontent-ref %}

{% content-ref url="modbus-rtu-protocol/read-write-memory-double-register-write-instruction-30" %}
[read-write-memory-double-register-write-instruction-30](https://docs.lorric.com/qr/fp-as510-paddle-wheel-flowmeter-manual-chinese-v1/communication/modbus-rtu-protocol/read-write-memory-double-register-write-instruction-30)
{% endcontent-ref %}