| Byte Offset | Field | Description | Typical Value | |-------------|-----------------|-------------------------------------------------|----------------| | 0 | Start of Header (SOH) | Delimiter | 0x02 | | 1 | Destination Address | Target device ID | 0x01 to 0xFF | | 2 | Source Address | Originating node (often A) | 0x41 ('A') | | 3 | Function Code | Command identifier | 0x420 (or 0x01A4) | | 4 | Register Start | Analog input register to read | 0x0000 | | 5 | Register Count | Number of points requested | 0x0001 | | 6 | Checksum | Simple XOR or CRC-8 | Variable | | 7 | End of Transmission (EOT) | Terminator | 0x03 |
The "420" and "421" often denote function codes or message identifiers. In many documented legacy systems, the serves as a read request for analog input registers, while the 421 packet acts as the corresponding acknowledgment and data payload . A-420 421 Packet
The represents a fascinating remnant of the early days of industrial data communication—a compact, function-specific message pair that bridged the analog and digital worlds. While largely superseded by more robust and open standards, it remains operational in thousands of legacy systems worldwide. | Byte Offset | Field | Description |
If you are looking for information related to these specific numbers, they often appear in the following professional contexts: Legal Citations: While largely superseded by more robust and open