Physically transmitting binary counter data ... how far?

Working on a project that will require receiving data from several (possibly 16 to 20) separate, remote 8-bit counters.
How long can a ribbon cable be, carrying the signal straight from a TTL or LS or even a CMOS (5v) circuit?
Uhhhh ... I think I've worded that correctly...
TIA
John R.
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JP Rahn wrote:

You can reach anywhere on the Earth with about 12,000 miles of cable, so that would be a reasonable goal. About 1260 uF of cable capacitance and a 50 ohm source would give a 63 mSec time constant. Call it 5 tau for good settling, and you get about a 3 Hz clock rate for the counters. Getting 00000000 gauge ribbon cable might be a tad pricey, tho.
Scaling down somewhat, there is no problem with many, many feet of ribbon cable as long as you protect I/O lines against ESD and the like, keep the clock rates low, use a hefty RC filter plus schmitt trigger at the inputs, and prevent any ground offsets between modules. That's basic signal conditioning that applies to any line driver/line receiver topology. CMOS is much easier to interface to than TTL, since it doesn't have any annoying input bias current to throw things off and it drives symmetrically. If you stay within a begnign environment and under a foot or two, you might get away with just alternating grounds with signals on the cable to sort of shield things, but once you go outside of the box you need to add basic protection and signal conditioning, just like RS-232 and other line driver interfaces. Once you do that, 0/+5V or +/-12V is just a matter of degree in noise immunity. We used to drive a thousand feet of umbilical cable to an underwater ROV with 5V levels - no problems as long as the signal was conditioned.
Gerry
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Why not use a "multidrop" serial bus, like I2C for that ? There are philips I2C counter chips available for that, also parallel/I2C converter chips. SPI would work as well, i believe.
-kert
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