I ran into some troubles trying to design a wired bus which will connect in about 64 testing devices. My first choice of mind was CAN bus, known of it's reliability and low EMC interference. However, during my research, I have discovered that there is limitation of minimum distance between CAN nodes, related to load capacitance and media capacitance: http://www.ti.com/lit/an/slla270/slla270.pdf
The nodes are connected through PCB traces and connectors, I have estimated media capacitance of 0.62 pF/cm (single 1 mm width trace to ground plane through 2.1 mm FR-4, is it right way to calculate?) and TJA1040 transceiver has input capacitance of max 10 pF.
My bus layout is:
Node to node distance is about 60 mm. From the graph, minimum distance is about 180 mm.
I also have some problems understanding bus characteristic impedance. In my design, I can manipulate CAN_L and CAN_H traces width and spacing, minimum width and spacing is 0.5 mm. With 2.1 FR-4 thickness, trace width of 1 mm and spacing of 0.5 mm (possible to manufacture for me) I can achieve 120 Ohm differential impedance, which is.. required, yes?
CAN Bus is a differential bus. Each differential pair of wire is a transmission line.Basically, the terminating resistor should match with the Characteristic Impedance of the transmission line to avoid reflection. CAN bus have a nominal characteristic line impedance of 120Ω. Due to that we are using typical terminating resistor value of 120Ω at each end of the bus.
I am asking this question because I wonder how it's (differential impedance) related to media capacitance. If I could manipulate media capacitance, I could lower minimum distance and maybe it would work with CAN.
If there's no possibility to make it work with CAN, what serial interface would you suggest instead of? Would RS-485 be better choice?