long cable capacitance consideration when the output is tri-stated

Question

I have 50 devices that are sending data to one MCU through multiple 3 meter data cables. The data signal lines merge at the MCU location. Only one device will send data at a time at 5 MHz+ data rate. (All devices outputs are in tri-state/hi-Z except the device that is sending data). I ran a test for a total of 3 devices and it works great. And not sure if I would run into data integrity issue due to enlarged total cable lengths when the number of device increases.

The question is whether I need to consider the capacitance of all the cables or just one cable for data integrity at high data rate in this setup? If all cables' capacitance need to be considered, I could use a mux to select a data line at a time. What are the other options?

• The data lines in the diagram are MISO lines of an SPI interface at 3.3 V logic level. It can be any device.
• It is the MCU board that I am designing.
• Extra components on the device ends should be avoided in this case, because 3 metre cables are not an issue at the moment for 3 devices. If they were longer cables, then I might choose to add RS-485 driver/receiver.

Answer 1

Most likely you run into capacitance issues.

Assuming the cable is CAT5, it has about 52 pF/m of capacitance. Fifty segments of 3 meters each is 7.8 nF.

That is a relatively high capacitance for a 5MHz+ data signal.

If you consider other options than a mux, slow down the speed of communication significatly. Or use a proper bus like RS-485, although it only supports a linear bus, and the star bus is not recommended.