RS485:
- 為半雙工(同一時間只能選擇傳送或接收的動作)
- 為差動傳輸，使用纜線兩端(Ｄ＋、Ｄ－)的電壓差來表示傳遞訊號
- 常用於單機傳送(Master)，多機接收(Slave)的通訊鏈結。可將數個感應裝置的數值回傳給Master
- Modbus是一種串列通訊協定
- Modbus為master/slave架構。一個裝置為Master節點，其他節點為slave。
- 每一個slave裝置都有一個唯一的位址/ID (1~247)
- Since a single byte is normally used to define the slave address and each slave on a network requires a unique address, the number of slaves on a network is limited to 256. The limit defined in the modbus specification is even lower at 247.
- master和slave之間走的Modbus 協定其封包格式有分ASCII 或RTU兩種
- ASCII 為文字模式, 用character 傳送, RTU 為binary 模式傳送
- 封包格式: Slave ID | Function Code | Data | CRC
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| RTU 訊息格式 |
- Information is stored in the Slave device in four different tables.
- Two tables store on/off discrete values (coils) and two store numerical values (registers). The coils and registers each have a read-only table and read-write table.
- Each table has 9999 values.
Each coil or contact is 1 bit and assigned a data address between 0000 and 270E.
Each register is 1 word = 16 bits = 2 bytes and also has data address between 0000 and 270E.
Coil/Register Numbers
|
Data Addresses
| Type | Table Name |
1-9999
|
0000 to 270E(9999)
| Read-Write | Discrete Output Coils |
10001-19999
|
0000 to 270E
| Read-Only | Discrete Input Contacts |
30001-39999
| 0000 to 270E | Read-Only | Analog Input Registers |
40001-49999
| 0000 to 270E | Read-Write | Analog Output Holding Registers |
Coil/Register Numbers can be thought of as location names since they do not appear in the actual messages. The Data Addresses are used in the messages.For example, the first Holding Register, number 40001, has the Data Address 0000. The difference between these two values is the offset.
Each table has a different offset. 1, 10001, 30001 and 40001.
只有Data Address(Offset)才出現在送出的Message 中, 如何知道查的是那一張表呢?
答案是利用Function Code 來決定
What is a function code?
The second byte sent by the Master is the Function code. This number tells the slave which table to access and whether to read from or write to the table.
Function Code (Command) 3 is to read 4xxx registers, and 4 for 3xxx registers.
| Function Code | Action | Table Name |
| 01 (01 hex) | Read | Discrete Output Coils |
| 05 (05 hex) | Write single | Discrete Output Coil |
| 15 (0F hex) | Write multiple | Discrete Output Coils |
| 02 (02 hex) | Read | Discrete Input Contacts |
| 04 (04 hex) | Read | Analog Input Registers |
| 03 (03 hex) | Read | Analog Output Holding Registers |
| 06 (06 hex) | Write single | Analog Output Holding Register |
| 16 (10 hex) | Write multiple | Analog Output Holding Registers |
Example:
Read Holding Registers (FC=03)
RequestThis command is requesting the content of analog output holding registers # 40108 to
40110 from the slave device with address 17.
11 03 006B 0003 7687
11: The Slave Address (11 hex = address17 )
03: The Function Code 3 (read Analog Output Holding Registers)
006B: The Data Address of the first register requested.
( 006B hex = 107 , + 40001 offset = input #40108 )
0003: The total number of registers requested. (read 3 registers 40108 to 40110)
7687: The CRC (cyclic redundancy check) for error checking.
Response
11 03 06 AE41 5652 4340 49AD
11: The Slave Address (11 hex = address17 )
03: The Function Code 3 (read Analog Output Holding Registers)
06: The number of data bytes to follow (3 registers x 2 bytes each = 6 bytes)
AE41: The contents of register 40108
5652: The contents of register 40109
4340: The contents of register 40110
49AD: The CRC (cyclic redundancy check).
Read Coil Status (FC=01)
Request
This command is requesting the ON/OFF status of discrete coils # 20 to 56
from the slave device with address 17.
11 01 0013 0025 0E84
11: The Slave Address (11 hex = address17 )
01: The Function Code 1 (read Coil Status)
0013: The Data Address of the first coil to read.
( 0013 hex = 19 , + 1 offset = coil #20 )
0025: The total number of coils requested. (25 hex = 37, inputs 20 to 56 )
0E84: The CRC (cyclic redundancy check) for error checking.
Response
11 01 05 CD6BB20E1B 45E6
11: The Slave Address (11 hex = address17 )
01: The Function Code 1 (read Coil Status)
05: The number of data bytes to follow (37 Coils / 8 bits per byte = 5 bytes)
CD: Coils 27 - 20 (1100 1101)
6B: Coils 35 - 28 (0110 1011)
B2: Coils 43 - 36 (1011 0010)
0E: Coils 51 - 44 (0000 1110)
1B: 3 space holders & Coils 56 - 52 (0001 1011)
45E6: The CRC (cyclic redundancy check).
The more significant bits contain the higher coil variables. This shows that coil 36 is off (0) and 43 is on (1). Due to the number of coils requested, the last data field1Bcontains the status of only 5 coils. The three most significant bits in this data field are filled in with zeroes.
RS485 模組 (MAX485)的Pin 腳定義:
RS485 模組 (MAX485)的Pin 腳定義:
- pin 1 :RO (receive out)
- pin 2 RE (receive enable)
- pin 3 :DE (data enable)
- pin 4 :DI (data in)
- pin 5, pin 8 : Gnd and Vcc onnected
- pin 6,7 : A and B the RS485 pair
RO------->UART TX
DI<------- UART RX
把/RE and DE 短路, 然後MCU/Pi 用一根GPIO 決定 /RE 或 DE.(即是決定是在送還是在收的狀態, 因為RS485是單工)
須考慮 Byte ordering的問題!
http://www.61131.com/download.htm
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