I have been browsing the net for a while to no avail: can somebody explain to me (and if not, give me a reference where I can find an answer) the relation between high wire capacity and the placement of pull-up resistors on that wire to compensate? Thanks and sorry for posting this question here
A couple of members of the forum at http://www.8052.com are very strong in this stuff, try posting there. Do note that at that forum, fortunately, real name is required, you will get bumped off if you try your thorny monniker.
I, when using long wires place a terminating resistor at either end with the resistnce of the wires characteristic impedance. RS485 is 'my' typical example.
Erik
But you have to separate pull-up resistors used in asymetric drive and a termination resistor to avoid problems with signal reflection.
A pull-up should be low enough to give the required rise times, while not so small that it generates undue power loss or transmission of high-frequency noise from the cable. If the goal is to limit the bandwidth, then it is preferable to use EMI filters in series with the signal. Since a voltage source is needed, most designs can only place the pull-up near the transmitter, but in some situations the serial resistance of the wire in relation to the individual capacitance sources nay need to be taken into account. In some situations, the majority of the capacitance is in the cable itself, but the individual listeners may have a large capacitance, in which case it may be advantageous to have a pull-up close to the listeners.
A terminating resistor must always match the characteristic impedance of the cable, so both placement and value are fixed.
Why do you ask? Are you (ab)using any serial communication close to - or past - the supported limitations regarding baudrate and/or cable length?
Thanks. I am asking because I am curious - a client built a too long a CAN network which malfunctioned, and the solution was to install pull-ups.