From 921770d03546462917e2fc312e1d5487c8dde42c Mon Sep 17 00:00:00 2001
From: Claas <claas@claas.dev>
Date: Wed, 19 Nov 2025 23:04:42 +0100
Subject: [PATCH] Experiment with different idea

---
 fan-controller/src/modbus/function/mod.rs     |  1 +
 .../modbus/function/write_holding_register.rs | 28 +++++++++++++++++++
 2 files changed, 29 insertions(+)
 create mode 100644 fan-controller/src/modbus/function/write_holding_register.rs

diff --git a/fan-controller/src/modbus/function/mod.rs b/fan-controller/src/modbus/function/mod.rs
index 1edfdce..ff2183f 100644
--- a/fan-controller/src/modbus/function/mod.rs
+++ b/fan-controller/src/modbus/function/mod.rs
@@ -1,5 +1,6 @@
 pub(super) mod code;
 pub(crate) mod read_input_register;
+pub(crate) mod write_holding_register;
 
 pub(crate) trait Function {
     const CODE: u8;
diff --git a/fan-controller/src/modbus/function/write_holding_register.rs b/fan-controller/src/modbus/function/write_holding_register.rs
new file mode 100644
index 0000000..ed5242a
--- /dev/null
+++ b/fan-controller/src/modbus/function/write_holding_register.rs
@@ -0,0 +1,28 @@
+use crate::modbus;
+
+/// Originally an experiment at a simpler data type to represent a modbus message
+pub(crate) struct WriteHoldingRegister([u8; 8]);
+
+impl WriteHoldingRegister {
+    pub(crate) fn new(device_address: u8, register_address: u16, value: u16) -> Self {
+        let register_address = register_address.to_be_bytes();
+        let mut data = [
+            device_address,
+            modbus::function::code::WRITE_SINGLE_REGISTER,
+            register_address[0],
+            register_address[1],
+            (value >> 8) as u8,
+            value as u8,
+            // CRC set in next step
+            0,
+            0,
+        ];
+
+        let checksum = modbus::CRC.checksum(&data[..6]).to_be_bytes();
+
+        // They come out reversed (or is us using to_be_bytes reversed?)
+        data[6] = checksum[1];
+        data[7] = checksum[0];
+        Self(data)
+    }
+}
-- 
2.51.2