# Electronic – Bus interface for short distances, many device data exchange – CAN, RS485? + characteristic impedance and media capacitance

buscanimpedanceNetwork

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.

How I calculated media capacitance:

Differential capacitance:

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?