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EspSoftwareSerial keeps two receive buffers: the byte buffer and an ISR buffer with one 32-bit entry per bit transition. We pass the same capacity for both, but a byte at 8E1 produces up to 11 transitions, so a 256 entry ISR buffer holds roughly 25 bytes, about 0.1 s of data at 2400 baud. Any loop() stall longer than that (serving the web UI, an MQTT publish burst, a LittleFS write) overflows the ISR buffer and the reader reports "Serial buffer full" even though the byte buffer is almost empty. Scale the ISR buffer by SWSERIAL_ISR_BUFFER_FACTOR (default 4: ~0.4 s of slack, 1 kB extra with the default 64 byte buffer). The library default of 11 would be correct but costs 11 kB with a 256 byte buffer, which ESP8266 cannot afford. The factor can be overridden at build time. Verified on an ESP8266 reading an Aidon 6525 over GPIO5 (software serial): with the factor at 8, loading the web UI over a weak WiFi link no longer produces serial buffer overflows.
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Problem
EspSoftwareSerial keeps two receive buffers: the byte buffer and an ISR buffer with one 32-bit entry per bit transition.
PassiveMeterCommunicatorpasses the same capacity for both. A byte at 8E1 produces up to 11 transitions, so a 256 entry ISR buffer holds roughly 25 bytes, about 0.1 s of data at 2400 baud. Anyloop()stall longer than that (serving the web UI, an MQTT publish burst, a LittleFS write) overflows the ISR buffer and the reader reports "Serial buffer full" although the byte buffer is almost empty. The library's own documentation suggestsbufCapacity * (2 + data bits + parity).Fix
Scale the ISR buffer by
SWSERIAL_ISR_BUFFER_FACTOR(default 4: about 0.4 s of slack, 1 kB extra with the default 64 byte buffer). The library default of 11 would be exact but costs 11 kB with a 256 byte buffer, which ESP8266 cannot afford. The factor can be overridden with-D SWSERIAL_ISR_BUFFER_FACTOR=n.Tested
ESP8266 reading an Aidon 6525 (2400 baud 8E1) on GPIO5 via software serial over a weak WiFi link. Before: loading the web UI reliably produced "Serial buffer overflow" in the debug log. With the factor raised, the same test produced none.