Well, he said 4 ft, so that would be more trouble than 1ft. I did have either a 12 or 20 ft Centronics cable, but that is a slow interface.
What speed SPI? We used ADCs ADuC8xx series and had horrible results at 1 ft until we used twisted pairs. It's been a few years but I think it was set for 10Mbps but was still flaky at 250Kb, the lowest speed it had. Only choices were 1M, 10M and 250K.
I would still not recommend SPI or I2C for this. It is always better to used something intended for the use that you have and neither of these does.
I think something else must have been wrong. I just ran a 5MHz square wave through 3ft of speaker cable, driven from a 50 Ohm generator. The signal comes out good. Overshoot is 17% and no degradation of risetime (better than
5ns). when terminated with 100 Ohm, the overshoot disappears. The same signal driven from a TTL gate shows 25% overshoot.
So with a bit of a decent driver and termination, the OP can get good results with simple single ended terminated wire and a 50 Ohm driver. RS-485 or CAN would be overkill.
does.
And when is something intended for the use? IMO anything that gives a good reliable result.
You are correct. I have the Altera Byteblaster on the 10 ft cable. Doing the SPI both directions by bit banging at ~1.5 MHz. No problem.
It depends. Once I was surprised to see the remote sensor with the Dallas 1-wire interface operating OK over ~15m of the twisted pair; however I've also seen the I2C not working on the same PCB with the SMPS because of the problematic layout.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
Typically in flat cable busses every other connector carries some signal and every other conductor is connected to a signal ground. With a signal ground on both sides of the signal conductor, there is a well defined characteristic impedance on the line (unless there are metallic surfaces close to the cable), so it is possible to build a matched system with no over/undershots.
However, any external magnetic or electric field can induce noise to the connection, since the signal and ground wires are not twisted around each other that would cancel some of the induced noise.
The construction also requires that no other DC or mains frequency currents are flowing in the signal ground conductors. In some cases this has required strapping together the frames of the CPU and the disk cabinet with 5-10 cm wide copper braids :-) in order to avoid problem caused by potential differences in the mains neutral wiring.
While the input and output pins are labeled as "TTL compatible" on most modern digital chips, have you actually looked at the input and output pin characteristics for input and output impedance and how well it matches the characteristic impedance of the flat cable (typically slightly below 100 ohms) ?
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