Re: Need a bus driver solution

"mmm" <pippo@xxxxxxxx> wrote in message news:dci2b5$cqj$1@xxxxxxxxxxxxxxxx
> Robert Latest wrote:
>> Well, it ain't exactly a bus driver, but anyway.
>> I have 20 TTL signals that I want to transmit over thirty feet of
>> cable. Max frequency on any of them is 500kHz. Timing isn't that
>> crucial because these are step/direction signals for a bunch of
>> stepper motor drives, so what's essentially important is only
>> that the number of pulses that make it to the other end stays the same.
>> Ideas:
>> 1) Use +/- 5V bipolar, single ended RS232-style technology. Found that
>> the ancient 1488-type chips don't seem to support that high a data rate,
>> although they're spec'd at 30V/us slew rate.
>> 2) 20 parallel symmetric twisted pairs. 3) Some form of serialization.
>> 1) and 2) are equally simple and straightforward, except that I'd like to
>> make do with at most a 25-pin DSUB connector which rules out 2). As for
>> 1), with 20 single-ended signals running in parallel over a cheap cable I
>> think crosstalk may become unmanageable. Or the thing would have to
>> become so low-impedance that power consumption becomes an issue.
>> 3) I've thought about sampling the inputs at a 1MHz rate,
>> stuffing them into three 8-bit shift registers and clocking the
>> data out at 8 MHz over symmetric twisted pairs. That would make
>> five pairs (three for the data, one clock and one strobe).
>> But before I begin working on this I'd like to know if there exist
>> industry-standard, integrated solution for this rather generic problem.
> I will do with rs485 of rs422 differential driver recieiver pairs if I
> want a robust proven solution ( a bit pricey ) the 422 drivers-receivers
> are done in 4 element in a package too, 2631 2632 pairs and other see on
> producers web sites ( texas national and others ) but rules 2 .... :-(
> a lower cost solution can be a standard 74xxx bus driver ( 244 245 )
> select a high current family ALS ACT and terminate the line with
> resistors but you have to do some test to see if they work
> long ago there was some chip for old mainframe bus ( 8T-sometihng made by
> national ) with long drive capabilities I don't konw if still available
>> Solutions that attack the motor connection at another point (i.e., not at
>> the step/direction level) are out because this is ancient equipment which
>> works very well. At the moment the motor drives are next to the control
>> computer (i386) with a bunch of thick cables running to the motors, and
>> I'd like to put the drives next to the motors and control the thing over
>> just one thin cable.
>> Thanks,
>> robert
Just from personal experience, a few rambling suggestions/thoughts that
might help ...
- use differential because then you don't need to worry about grounding
quite so much
- use twisted pair ribbon cable - one 50way will provide 25 pairs, expensive
but worth it
- send with five 26C31 quad RS285/422 drivers
- include a 33R series resistor at the driver end of each hi/lo to limit
instantaneous current output
- means the drivers won't try to create really fast edges in the cable
- receive with five 26C32 quad receivers, terminate lines with around 500R
- the termination means that a few microamps of noise will not induce much
voltage
- buffer received signals with four 74HCT14 hex inverters
- put a tiny RC filter (100R, 100pF ?) between 26C32 and 74HCT14 to filter
glitches
- 74HCT14 has genuine hysteresis which will noise glitch sensitivity
- feeding ribbon pairs through ferrite beads will slug really fast edges a
bit
- include a 33R series resistor at the driver end of each hi/lo to limit
instantaneous current output
- means the drivers won't try to create really fast edges in the cable
- make sure that the 0V reference between drivers and receivers goes direct
fromthe devices
- not via faraway powerlines - use the 10 spare ribbon lines for this ?
- power the receiver circuit from a separate supply
- means no worries about chunky power currents going to/from remote
circuit

general point
Are costs constraints involved ?
who is paying for the parts ? will an extra few dollars be significant
how is your time costed ? home work may be free, company may want least time
spent on job
- it may be best to spend a few dollars extra on components / connectors
than to spend time debugging or rebuilding. All depends on your individual
circumstances.

hope this helps,
Neil
(old codger engineer)



.

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