Transport Selection Guide

UART / RS-485

Most common transport. Call ostx_stream_pack() (or ostx_static_pack()) inside loop() or an ISR trigger.

OSTX_STATIC_DEFINE(s_t, AID, "T1", "Cel");
static void emit(ostx_u8 b, void*) { Serial.write(b); }

void loop() {
    ostx_stream_pack(&s_t, tid++,
                     (ostx_u32)(millis()/1000), scaled, emit, NULL);
    delay(1000);
}

For distances > 10 m, use a MAX485 or SN75176 RS-485 transceiver. The OSynaptic wire format is self-delimiting (no \n required).

WiFi UDP (ESP32 / ESP8266)

Use ostx_static_pack() to build a buffer, then send as a UDP datagram:

#include <WiFiUdp.h>
WiFiUDP udp;
static ostx_u8 buf[OSTX_PACKET_MAX];

void send_reading(void) {
    int n = ostx_static_pack(&s_temp, tid++,
                              (ostx_u32)(millis()/1000), scaled, buf);
    if (n > 0) {
        udp.beginPacket(SERVER_IP, SERVER_PORT);
        udp.write(buf, (size_t)n);
        udp.endPacket();
    }
}

Set OSTX_NO_STREAM=1 to save 160 B Flash when streaming is not needed.

LoRa (SX1276 / SX1278 / SX1262)

Use ostx_static_pack() to build the frame, then send with the LoRa library:

static ostx_u8 buf[OSTX_PACKET_MAX];

void send_lora(void) {
    int n = ostx_static_pack(&s_hum, tid++,
                              (ostx_u32)(millis()/1000), scaled, buf);
    if (n > 0 && LoRa.beginPacket()) {
        LoRa.write(buf, (size_t)n);
        LoRa.endPacket();
    }
}

LoRa payload limit: SX1276 max payload 255 bytes; OpenSynaptic frames are typically 20-40 bytes — well within budget.

nRF24L01+

Use ostx_static_pack() and split into 32-byte fragments if needed (OpenSynaptic frames are typically < 32 bytes for 1-2 sensors):

static ostx_u8 buf[OSTX_PACKET_MAX];

void send_nrf(void) {
    int n = ostx_static_pack(&s_co2, tid++,
                              (ostx_u32)(millis()/1000), scaled, buf);
    if (n > 0 && n <= 32) {
        radio.write(buf, (uint8_t)n);
    }
}

Standalone WiFi vs Integrated WiFi

Approach	MCU	WiFi module	Integration
Integrated	ESP32 / ESP8266	On-chip	Best — single firmware
AT command	ATmega328P	ESP-01(AT)	Use `Serial1` for sensor TX, `Serial` for AT channel
SPI bridge	ATmega328P	Wiznet W5500	Build UDP frame directly with Ethernet library

Transport Summary

Transport	Buffer mode	Typical latency	Range
UART	Stream (Tier C)	< 1 ms	1–15 m (RS-485: 1200 m)
WiFi UDP	Buffered (Tier B)	5–50 ms LAN	Unlimited (LAN/WAN)
LoRa	Buffered (Tier B)	50–500 ms	0.5–15 km
nRF24L01+	Buffered (Tier B)	1–5 ms	10–100 m
BLE (notify)	Stream (Tier C)	10–45 ms	10–30 m

UART / RS-485​

WiFi UDP (ESP32 / ESP8266)​

LoRa (SX1276 / SX1278 / SX1262)​

nRF24L01+​

Standalone WiFi vs Integrated WiFi​

Transport Summary​