resistores no max485
Hello,
I overlooked the pullup strength question. I use to have weak pullup/down, e. g. 10 k at every node, to prevent unconnected devices from detecting a break state, I think that's rather common.
For the end nodes, where the parallel termination, e. g. 120 ohms would be placed, some standards specify strong pullups and pulldowns. PROFIBUS has 390 ohms and replaces 120 ohms AB termination with 150 ohms, to have the nominal impedance with the combination of resistors.
Strong termination has the purpose of suppressing false start bits from signal reflections after disabling the transmitter. Cause PROFIBUS has a strict timing scheme for token rotation, false start bits could effectivly knock out the protocol.
Regards,
Frank