I am currently designing a device which internally consists of two circuit boards inside a fairly large, grounded metal enclosure. I'm designing one of the boards myself, the other is a bought part which communicates via UART over a pin header connector (ideal for connecting ribbon cable). I'd like to be able to use the 115200 baud speed, but that's not an absolute requirement.
The bought part is available in two versions - one with 3.3V UART and one with proper RS232 voltage levels. Using the 3.3V version would simplify my own board a bit (no need for a MAX(3)232), but I am wondering whether that might have a negative impact on the robustness of the transmission, especially since the device will have to pass an EMC test.
Since I'm a novice electronics designer, I don't have a good intuition for this yet. Do you think the 3.3V level is sufficiently robust over a ~20cm ribbon cable, provided I don't run it right next to the power supply?
Would you recommend putting a ferrite around both ends of the cable? The supply for the secondary board is over the same cable, so I'd expect the sum of currents over that cable to be 0 for all "good" signals, except for high frequency signals, which can run directly to the chassis ground via a 1nF capacitor.