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[modbus] Fix timeout for non-hardware UARTs (e.g., USB UART) (#14614)

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Co-authored-by: Clyde Stubbs <2366188+clydebarrow@users.noreply.github.com>
This commit is contained in:
J. Nick Koston
2026-03-08 14:31:42 -10:00
committed by GitHub
co-authored by Clyde Stubbs
parent 93d7ec4d72
commit 88536ff72b
8 changed files with 213 additions and 8 deletions
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esphome/components/modbus/modbus.cpp
+15 -3
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@@ -11,6 +11,11 @@ static const char *const TAG = "modbus";
// Maximum bytes to log for Modbus frames (truncated if larger)
static constexpr size_t MODBUS_MAX_LOG_BYTES = 64;
// Approximate bits per character on the wire (depends on parity/stop bit config)
static constexpr uint32_t MODBUS_BITS_PER_CHAR = 11;
// Milliseconds per second
static constexpr uint32_t MS_PER_SEC = 1000;
void Modbus::setup() {
if (this->flow_control_pin_ != nullptr) {
this->flow_control_pin_->setup();
@@ -19,10 +24,17 @@ void Modbus::setup() {
this->frame_delay_ms_ =
std::max(2, // 1750us minimum per spec - rounded up to 2ms.
// 3.5 characters * 11 bits per character * 1000ms/sec / (bits/sec) (Standard modbus frame delay)
(uint16_t) (3.5 * 11 * 1000 / this->parent_->get_baud_rate()) + 1);
(uint16_t) (3.5 * MODBUS_BITS_PER_CHAR * MS_PER_SEC / this->parent_->get_baud_rate()) + 1);
// When rx_full_threshold is configured (non-zero), the UART has a hardware FIFO with a
// meaningful threshold (e.g., ESP32 native UART), so we can calculate a precise delay.
// Otherwise (e.g., USB UART), use 50ms to handle data arriving in chunks.
static constexpr uint16_t DEFAULT_LONG_RX_BUFFER_DELAY_MS = 50;
size_t rx_threshold = this->parent_->get_rx_full_threshold();
this->long_rx_buffer_delay_ms_ =
(this->parent_->get_rx_full_threshold() * 11 * 1000 / this->parent_->get_baud_rate()) + 1;
rx_threshold != uart::UARTComponent::RX_FULL_THRESHOLD_UNSET
? (rx_threshold * MODBUS_BITS_PER_CHAR * MS_PER_SEC / this->parent_->get_baud_rate()) + 1
: DEFAULT_LONG_RX_BUFFER_DELAY_MS;
}
void Modbus::loop() {
@@ -290,7 +302,7 @@ void Modbus::send_next_frame_() {
this->last_send_tx_offset_ = 0;
} else {
this->write_array(frame.data.get(), frame.size);
this->last_send_tx_offset_ = frame.size * 11 * 1000 / this->parent_->get_baud_rate() + 1;
this->last_send_tx_offset_ = frame.size * MODBUS_BITS_PER_CHAR * MS_PER_SEC / this->parent_->get_baud_rate() + 1;
}
#if ESPHOME_LOG_LEVEL >= ESPHOME_LOG_LEVEL_VERBOSE