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I have here a PCB which has a kind of "open collector" RS-485, known as J1708 (schematic attached). The default baud rate is 9600 bps and there is no terminators, J1708-TX is a 3.3V signal. I would like to know if I can get higher baud rates like 38400, 57600 and maybe 115200 bps with the same schematic but changing (reducing...) the values of resistors R1 and R2 and capacitors C1 and C2... enter image description here

abomin3v3l
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  • The only way to answer this is to try it and monitor the signal waveforms. What happened when you did that? Besides which, what do you hope to communicate with at a nonstandard baud rate? – Dave Tweed Mar 27 '19 at 17:01
  • The monopolar drive and passive return, with cable and terminating capacitance will probably limit you to something near what the standard requires. – KalleMP Mar 27 '19 at 21:30

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If the Wikipedia article on J1708 is to be believed, then it's only J1708 if it's going at 9600 baud.

You may be able to make it work over short distances at higher baud rates, but I doubt you'll be able to make it work over J1708's specified 40m distance -- and regardless, it wouldn't be J1708 any more.

TimWescott
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  • I have just removed C1 and C2, and changed R1 and R2 to 680 ohms on the boards (master and slaves) an it is working at 115200 bps, I need a cable lenght of 5 meters at maximum. And at 115200 bps the signals of A and B are pretty well (almost square waves) on the transceivers when they are receiving or transmitting... – abomin3v3l Mar 28 '19 at 17:37
  • Be sure to test it in a similar noise environment to where it needs to work! – TimWescott Mar 28 '19 at 22:13