4.3 KiB

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Protocol

The protocol checksum is litle-endian CRC16-MODBUS, but the data itself seems to be big-endian (network order). It is structured around 16bit words (read 4 returns 8 bytes).

I have seen the "Transfer ID?" field containing 0xFF and 0x01, I am not yet sure of the putpose of this field. Most transactions it is set to 0xFF.

Reading

Read request

   ┌──────────────┐
 ╔═╡ Transfer ID? │
 ║ └──────────────┘
 ║    ┌────────────────────┐
 ║  ╔═╡ Operation (3=read) │
 ║  ║ └────────────────────┘
 ║  ║      ┌───────────────┐
 ║  ║    ╔═╡ Start address │
 ║  ║    ║ └───────────────┘
 ║  ║    ║       ┌────────────────────────────────┐
 ║  ║    ║     ╔═╡ Number of 2-byte words to read │
 ║  ║    ║     ║ └────────────────────────────────┘
 ║  ║    ║     ║       ┌─────────────────┐
 ║  ║    ║     ║     ╔═╡ Transaction CRC │
 ║  ║    ║     ║     ║ └─────────────────┘
┌╨─┬╨─┬──╨──┬──╨──┬──╨──┐
│FF│03│00 0C│00 08│91 d1│
└──┴──┴─────┴─────┴─────┘

Read response

   ┌──────────────┐
 ╔═╡ Transfer ID? │
 ║ └──────────────┘
 ║    ┌────────────────────┐
 ║  ╔═╡ Operation (3=read) │
 ║  ║ └────────────────────┘
 ║  ║    ┌──────────────────────────┐
 ║  ║  ╔═╡ Number of bytes returned │
 ║  ║  ║ └──────────────────────────┘
 ║  ║  ║    ┌──────┐                    ┌─────────────────┐
 ║  ║  ║  ╔═╡ Data │                    │ Transaction CRC ╞═╗
 ║  ║  ║  ║ └──────┘                    └─────────────────┘ ║
┌╨─┬╨─┬╨─┬╨──────────────────────────────────────────────┬──╨──┐
│FF│03│10│20 20 20 20 4D 4C 32 34 32 30 20 20 20 20 20 20│FD 17│
└──┴──┴──┴───────────────────────────────────────────────┴─────┘

This particular memory section contains the device SKU: ML2420

Writing

Write request

   ┌──────────────┐
 ╔═╡ Transfer ID? │
 ║ └──────────────┘
 ║    ┌─────────────────────┐
 ║  ╔═╡ Operation (6=write) │
 ║  ║ └─────────────────────┘
 ║  ║      ┌─────────┐
 ║  ║    ╔═╡ Address │
 ║  ║    ║ └─────────┘
 ║  ║    ║       ┌──────┐
 ║  ║    ║     ╔═╡ Data │
 ║  ║    ║     ║ └──────┘
 ║  ║    ║     ║       ┌─────────────────┐
 ║  ║    ║     ║     ╔═╡ Transaction CRC │
 ║  ║    ║     ║     ║ └─────────────────┘
┌╨─┬╨─┬──╨──┬──╨──┬──╨──┐
│FF│06│01 0A│00 01│7C 2A│
└──┴──┴─────┴─────┴─────┘

The data at 0x010A is a boolean controlling the load output switch.

Write response

Seems to return exactly the same as the data written, presumably to allow verifying that the data did indeed get written.

4.3 KiB Raw Blame History