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RS232 (EIA-232, TIA-232-F) standard allows signal level +-3 .. 15V operating signal level voltage on the line.
Also: [Indefinite short circuit to ground or to any voltage level up to ±25 volts]. For 3.3V systems (BT-RS232 adapters and some STM32 devices), ST3232/MAX3232 chip is used as line drivers. 3 types of linedrivers were previously used in BT-RS232 and STM32: * ST3232 * MAX3232I (Texas, with [Texas Instruments logo]) * MAX3232 non-Texas (branded Maxim, without Texas Instruments logo) ** NOT used any longer, not sold (withdrawn or replaced; however, all 3 types were used in 2018) All pass tests reproducibly with FTDI USB-RS232 adapters, and VEMS ECU with texas MAX232 (or MAX232I). * The non-Texas MAX3232 type was found to sometimes work, but sometimes not with some VEMS ECU with ST232. ** Since it works with most other peers, it might falsely appear that the communication issue is related to (newer, with ST232) VEMS ECU-s. ** However, these VEMS ECU-s with ST232 have the cleanest, nicest, sharpest (highest baudrate capable) RS232 signal of all, appr +-9V (perfectly within the range allowed by the RS232 standard, but slightly greater than the +-8V of Texas MAX232 used in older VEMS ECU) ** non-Texas MAX3232 also works reliably if: RX=10k pulldown and 4k7 (series resistor is used) from the TX signal of VEMS ECU (otherwise perfect, standard compliant output)... in other words, divided by 1.5: 9V/1.5=6V ** although that voltage divider is valid for other signals as well, easy to apply (2 resistors), does no harm if applied and forgotten, it was quite shocking to find out that the non-Texas MAX3232 needed it, because RS232 side RX was somehow sensitive to greater than +8.8V ... that is why it is NOT used any longer, and was removed from PCB-s in stock ** ST3232 or Texas MAX3232 chips are offered to customers free of charge (use order comment eg.: "add 3 ST3232 or Texas MAX3232 +some 4k7-10k resistors") in case someone suspects he might have some from old purchase (kept in drawer, or moved from an install where it worked fine, to a new install where effected, because the ECU has nicer RS232 signal). Sorry about the inconvenience USB is known for occasional compatibility problems (mostly windows drivers, but sometimes more tricky, involving supply current rating, with USB hubs or not), compatibility issues with 3 line-RS232 devices (where DTR, RTS, ... signals are not used, only RX,TX and GND) have been almost nonexistent and unheard of... (except forgotten ground or miswiring) ---- See also |