Prevent unintentional loopback (staying in "boot mode" if RS232 is not connected to BT-RS232 or PC at powerup). Obligatory if DSUB9 extension cable is used

Capacitive coupling can cause unintentional loopback detected (very likely with long RS232 cable,less likely with short cable or no cable, but not impossible), staying in bootloader with stayreason 0x37 (instead of starting firmware).

To prevent unintentional loopback: obligatory when long DSUB9 extension cable is used: (but Vemstune or BT-RS232 is not connected)

* connect a DSUB9/M connector with resistor < 5.6k between RS232DSUB9F/pin3 (ECU RX) and RS232DSUB9F/pin5 (GND).

** it might be tempting to insert a 2k4 throughole resistor and forget about it. It's OK as a test, but not a permanent solution: expected to lose contact.

Summary

* resistor between ECU DB9F/pin3 and pin5 prevent unintentional loopback 2k7 recommended, but 0 Ohm ("short") wire also works.

** This way capacitive coupling (or other noise) cannot cause unintentional loopback. Note that with some random ECUs (having ST232, from 2018-October mainboard batch) we measured 4.17V with 1k pullup to 5.05V indicating 4.17 / (5.05-4.17) = 4.73 k internal pulldown inside ECU (actually inside ST232 chip) which makes unintentional loopback extremely unlikely without real strong capacitive coupling, such as a long DSUB9 extension with no peer device connected. Confirmed by:

** We made tests with long (5 * 1.5m) DB9 cable extension on RS232: ECUs (having ST232) from current mainboard batch (2018-10 picknplace batch): started fw reliably even such abnormally long total 7.5m long cable (capacitive loopback) was attached, unterminated.

* resistor between pin3 and pin2 at powerup causes intentional loopback (see top of this page)

** usually not needed for fw upgrade, or done temporary with a lose resistor (before firmware upload if needed)