feat(slave_node): Implement initial Modbus RTU server

- Add a basic Modbus RTU server implementation based on Zephyr samples.
- Configure usart1 for Modbus via a board overlay.
- The server initializes and runs, but polling with mbpoll results in a timeout.
- This commit captures a functional but non-working state for further debugging.

software/apps/slave_node/boards/bluepill_f103rb.overlay

@@ -0,0 +1,7 @@
+&usart1 {
+	modbus0 {
+		compatible = "zephyr,modbus-serial";
+		status = "okay";
+	};
+	status = "okay";
+};

software/apps/slave_node/prj.conf

@@ -1,6 +1,6 @@
 # Enable Console and printk for logging
 CONFIG_CONSOLE=y
-CONFIG_PRINTK=y
+CONFIG_LOG=y
 
 # Disable UART console
 CONFIG_UART_CONSOLE=n
@@ -8,3 +8,8 @@ CONFIG_UART_CONSOLE=n
 # Enable RTT console
 CONFIG_RTT_CONSOLE=y
 CONFIG_USE_SEGGER_RTT=y
+
+# Config modbus
+CONFIG_UART_INTERRUPT_DRIVEN=y
+CONFIG_MODBUS=y
+CONFIG_MODBUS_ROLE_SERVER=y

software/apps/slave_node/src/main.c

@@ -1,16 +1,115 @@
 /*
- * Copyright (c) 2025 Eduard Iten
+ * Copyright (c) 2020 PHYTEC Messtechnik GmbH
+ * Copyright (c) 2022 Nordic Semiconductor ASA
  *
  * SPDX-License-Identifier: Apache-2.0
  */
 
 #include <zephyr/kernel.h>
+#include <zephyr/sys/util.h>
+#include <zephyr/drivers/gpio.h>
+#include <zephyr/modbus/modbus.h>
+#include <zephyr/usb/usb_device.h>
+
+#include <zephyr/logging/log.h>
+LOG_MODULE_REGISTER(mbs_sample, LOG_LEVEL_INF);
+
+static uint16_t holding_reg[8];
+static uint8_t coils_state;
+
+static int coil_rd(uint16_t addr, bool *state)
+{
+	*state = true;
+
+	LOG_INF("Coil read, addr %u, %d", addr, (int)*state);
+
+	return 0;
+}
+
+static int coil_wr(uint16_t addr, bool state)
+{
+	bool on;
+	if (state == true) {
+		coils_state |= BIT(addr);
+		on = true;
+	} else {
+		coils_state &= ~BIT(addr);
+		on = false;
+	}
+
+	LOG_INF("Coil write, addr %u, %d", addr, (int)state);
+
+	return 0;
+}
+
+static int holding_reg_rd(uint16_t addr, uint16_t *reg)
+{
+	if (addr >= ARRAY_SIZE(holding_reg)) {
+		return -ENOTSUP;
+	}
+
+	*reg = 0xDEAD;
+
+	LOG_INF("Holding register read, addr %u", addr);
+
+	return 0;
+}
+
+static int holding_reg_wr(uint16_t addr, uint16_t reg)
+{
+	if (addr >= ARRAY_SIZE(holding_reg)) {
+		return -ENOTSUP;
+	}
+
+	holding_reg[addr] = reg;
+
+	LOG_INF("Holding register write, addr %u", addr);
+
+	return 0;
+}
+
+static struct modbus_user_callbacks mbs_cbs = {
+	.coil_rd = coil_rd,
+	.coil_wr = coil_wr,
+	.holding_reg_rd = holding_reg_rd,
+	.holding_reg_wr = holding_reg_wr,
+};
+
+const static struct modbus_iface_param server_param = {
+	.mode = MODBUS_MODE_RTU,
+	.server = {
+		.user_cb = &mbs_cbs,
+		.unit_id = 1,
+	},
+	.serial = {
+		.baud = 19200,
+		.parity = UART_CFG_PARITY_NONE,
+	},
+};
+
+#define MODBUS_NODE DT_COMPAT_GET_ANY_STATUS_OKAY(zephyr_modbus_serial)
+
+static int init_modbus_server(void)
+{
+	const char iface_name[] = {DEVICE_DT_NAME(MODBUS_NODE)};
+	int iface;
+
+	iface = modbus_iface_get_by_name(iface_name);
+
+	if (iface < 0) {
+		LOG_ERR("Failed to get iface index for %s", iface_name);
+		return iface;
+	}
+
+	return modbus_init_server(iface, server_param);
+}
 
 int main(void)
 {
-	while (1) {
-		printk("Hello from the updated Slave Node! Version 2.0\n");
-		k_msleep(1000);
+	LOG_INF("Starting APP");
+	if (init_modbus_server()) {
+		LOG_ERR("Modbus RTU server initialization failed");
 	}
+	LOG_INF("APP started");
 	return 0;
 }