[modbus] Speed up the bus with microsecond-accurate timing (#12421)

tests/components/modbus/common.h

@@ -11,7 +11,14 @@ namespace esphome::modbus::testing {
 // A UART that discards all writes, for tests that never inspect the wire.
 class NullUART : public uart::UARTComponent {
  public:
-  NullUART() { this->set_baud_rate(115200); }
+  // 8N1, matching what the uart schema emits for a real hub; the framing drives the modbus
+  // interframe timing, so leaving data/stop bits at their zero defaults would not be representative.
+  NullUART() {
+    this->set_baud_rate(115200);
+    this->set_data_bits(8);
+    this->set_stop_bits(1);
+    this->set_parity(uart::UART_CONFIG_PARITY_NONE);
+  }
   void write_array(const uint8_t *data, size_t len) override {}
   bool peek_byte(uint8_t *data) override { return false; }
   bool read_array(uint8_t *data, size_t len) override { return false; }

tests/components/modbus/modbus_framing_test.cpp

@@ -0,0 +1,64 @@
+#include <gtest/gtest.h>
+
+#include <cstdint>
+
+#include "common.h"
+#include "esphome/components/modbus/modbus.h"
+
+namespace esphome::modbus::testing {
+
+namespace {
+
+// Exposes the timing values setup() derives from the UART framing.
+class FramingProbeHub : public ModbusClientHub {
+ public:
+  uint32_t bits_per_char() const { return this->bits_per_char_; }
+  uint32_t frame_delay_us() const { return this->frame_delay_us_; }
+};
+
+class FramedUART : public NullUART {
+ public:
+  FramedUART(uint32_t baud_rate, uint8_t data_bits, uint8_t stop_bits, uart::UARTParityOptions parity) {
+    this->set_baud_rate(baud_rate);
+    this->set_data_bits(data_bits);
+    this->set_stop_bits(stop_bits);
+    this->set_parity(parity);
+  }
+};
+
+}  // namespace
+
+// 8N1 is 10 bits on the wire, so t3.5 at 9600 baud is 3.5 * 10 / 9600 = 3645.8us.
+TEST(ModbusFraming, EightNoneOneDerivesTenBits) {
+  FramedUART uart(9600, 8, 1, uart::UART_CONFIG_PARITY_NONE);
+  FramingProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();
+
+  EXPECT_EQ(hub.bits_per_char(), 10u);
+  EXPECT_EQ(hub.frame_delay_us(), 3646u);
+}
+
+// Spec-conformant RTU framing is 11 bits, which lengthens the interframe gap to
+// 3.5 * 11 / 9600 = 4010.4us, rounded up.
+TEST(ModbusFraming, EightEvenOneDerivesElevenBits) {
+  FramedUART uart(9600, 8, 1, uart::UART_CONFIG_PARITY_EVEN);
+  FramingProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();
+
+  EXPECT_EQ(hub.bits_per_char(), 11u);
+  EXPECT_EQ(hub.frame_delay_us(), 4011u);
+}
+
+// Above 19200 baud the spec's fixed 1750us floor governs instead of 3.5 characters.
+TEST(ModbusFraming, FastBaudUsesSpecFloor) {
+  FramedUART uart(115200, 8, 1, uart::UART_CONFIG_PARITY_NONE);
+  FramingProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();
+
+  EXPECT_EQ(hub.frame_delay_us(), 1750u);
+}
+
+}  // namespace esphome::modbus::testing