I'm involved in a project where the customer defined pins in a ribbon cable, without considering possible cross-talk issues. The signals are 1 MHz data signals with no ground wire separating them. I've never had experience with cross-talk and was amazed at the size of the induced glitches (0.5 to 0.65 volts). The receiving side was using 74HCxx line drivers (CMOS switching levels) which resulted in pure garbage on the data stream. The customer is switching to 74HCT drivers in an attempt to move the input "high" switching level below the glitch level, but I have my concerns.

Is the anything that can be done, besides switching to HCT parts or just properly redesigning the board to possibly salvage what we have?

Any comments welcome.

Thanks.

You can leave the board design as-is, but make a short adapter on both ends of the cable, and make the actual cable either as a non-ribbon cable (micro coax, this will be the best), or use proper grounding between signal wires. Essentially you need to make a different cable to fit the IDC plugs (or whatever they selected as board-to-cable connector). Something like this:

Can you change the ribbon cable, or insert an adapter to a higher pin-count cable? Consider what IDE/ATA did to increase bandwidth -- it was switched from a 40-wire cable to an 80-wire cable, with every other wire inside the cable tied to ground within the connector. A similar solution could apply here.

Alternatively, can you reduce the slew rate? At 1 MHz, your problem is likely to be less about the frequency of the signals themselves and more about their fast edges. A filter network on the transmit side may help.

After the fact, you have a few choices:

1. Use Schmitt trigger input receivers


2. use shielded foil ribbon cable


3. terminate with 470 pF as a starting value


4. terminate with cable impedance 110-120 Ohms to ground


5. terminate with driver impedance ~ 50 Ohms to Vcc/2 ore equiv pull/down

Increasing the source resistance reduces risetime but won't reduce crosstalk , because the impedance ratio of crosstalk capacitance Xc/Rs rises as slew rate of current reduces.

edit

Proof of ideas using 1m ribbon cable estimate ESL and C

Here using 5 different signals near 1MHz square wave but different to get alias crosstalk with different source and load impedances. Normally I recall, ribbon cables are 120 Ohm single ended which translates into a lump inductance and capacitance per meter but depends on AWG and dielectric spacing.