Need a bus driver solution

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

Ah, I found National's DS90CR215 and friends. Exactly what I need, and I can probably use cheap, store-bought CAT5 network patch cables. Downside is that I'll have to hand-solder TSSOP packages -- ugh, so far I've managed to stay away from SMD, but TSSOP is really terrible. 0.5mm picth! What a pitch!

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)

How about 20 5-volt cmos-driven lines and a couple of commons (ground) from the transmit end. At the receive end, use RS-485 line receivers, all sharing the low-side common lines. That should zap any common-mode noise. Hooks for some modest value caps across the line receiver outputs would be insurance against picking up fast normal-mode noise or crosstalk glitches (or RC-schmitt things, to be more politically correct.)

John

On 31 Jul 2005 10:59:28 GMT, Robert Latest wroth:

Serial would be my first choice too. You need to be aware that with the serial approach the data is sampled at the clock rate and you will lose some information involved with exactly where edges of the individual channel transitions occur with respect to each other. That may or may not be a problem for your application.

In other words, if two of your signals are never supposed to be high at the same time, there is a good probablity that because of the sampling they could overlap by as much as the period of one clock cycle. For a 20 MHz clock, that could be as much as a 50 nanosecond overlap glitch.

Jim

Ah, not such a good choice in a motor application. LVDS is low voltage for super high speeds at reduced signal swing, low power, and high density. The drawback is CMR is not so good, ranging in the 1V range. Any kind of RS-422 driver/receiver has much higher CMR rating. The TI SN75ALS194 and SN75ALS195 quad combos with +/-7V CMRR, plenty of speed, low power 5V supply operation, TTL compatibility, low cost, wide availability (Digi-Key), and DIP packaging, are a better option. Your line length is not so great that the savings in wire does anything for you.

Whew- a lot of signal loss into 100 ohm load there- better to put a ferrite clamp on cable.

Get over it- some people grow old and others simply get old.

That's not very aesthetic, but would probably work. That's one way to use D25 connectors and cheap RatShack RS-232 cables, where the wires inside aren't shielded from each other.

Oh, if you use cmos drive and a differential line receiver, you have to bias up the low side of the receiver 2.5 volts.

If you don't expect big low-frequency ground loops, just drive from cmos and receive with an r-c-schmitt, tau a couple hundred ns. That will filter out all sorts of sins.

John

You missed at least one idea. You could use 74HC244 buffers and 100 ohm resistor R-packs to drive the signals.

If you use 50 wire ribbon cable and ground every second line, getting

500KHz signals through 20 feet isn't the biggest problem. The really big problem is the grounding of the two ends. If the far end makes no connection to the outside world, your life is easy.

If both ends of the system connect to the mains wiring ground, you need to worry about voltage differences up to at least 2V.

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kensmith@rahul.net   forging knowledge

Nah- the major idea missed here is that "serialization" does not mandate bit serial. He could always do byte serial with latching strobe or some other equally simple approach to fit this on a 25 DSUB, while retaining the best driver technology for the job , RS-422.

[... good suggestions ...]

Not really since it'll be built only once or twice

It's my own company ;-)

Absolutely. I like high-quality but non-specialized stuff. I.e., DB25 connectors, but the really good spec'd stuff. That's why I was at first intrigued with the LVDS idea because it looked like a simple patch TP cable could have been used.

robert

I've thought about it; but at 500 kHz I'd like a rise time of

1us, which, with as much as 1000pF cable capacitance, drives 5mA into the adjacent line. Or do you suggest to drive the lines really hard, creating big but short spikes on the other lines which then get filtered out by the RC - Schmitt trigger combos?

robert

That's why I suggested 500 ohm terminators. No doubt it doesn't match the line impedance properly, but all I was thinking is that limiting the max current that is available to change the line level will reduce the noise radiated to other lines. I had a similar problem to sort out at work a couple of weeks ago. Tried a ferrite clamp around the ribbon but couldn't see any difference on the signal - the edges seemed just as sharp. But a ferrite bead with a couple of turns did appear to round the edges a bit. The pulses on these lines were

500nS so I had to limit noise without too much slugging. The 74HCT14 seems to do a pretty good job at ignoring signals wobbling a volt or so away from the rails, so I like them. And the differential driver/receiver seem to work well, with a terminator of up to 1K or so. That means (to me anyway) that less current is drawn at the driver and pumped up and down the wires, so less radiated effect. No doubt my empirical meanderings could be bettered, but it works ok in the lab test sets it's needed in. For the flight stuff we use a diff driver with opto-isolator and rs flipflop type cicruit to square it up (a properly designed serial link), and then run it at only 4Mhz. That's worked fine for a few years on quite a few aircraft ... hth, ymmv etc Neil

Our local council needs a bus driver solution too. IF you get in one of those busses and survive the journey without s*****ng your pants you are doing well.

Serial would be my first choice too. I just took a look at

A cheap, bulletproof, solution would be plastic fiber optical cable hooked up to a serializer on one end and a de-serializer on the other. The common mode ground loop could be megavolts with no problems!

Jim

Neil, you should study a bit the theory, there are a lot of mistakes and guesswork in your reply.

1.) the ferrite beads or clamps around the whole cable do *not* have any effect on rise times of the signals. The signal currents cancel with the return currents and so the clamp has *no* effect. It will have a high impedance for common mode disturbances, that is what it is used for. If you want it to have an effect on the signal, you will need to put a separate bead in all of the lines. 2.) transmission line theory should be a must if you work in this area. This would explain the "wobbling" and give you a good idea what to do to avoid it. As to the aircraft, I hope there is at least a good hardware engineer in the company to superwise.

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ciao Ban
Bordighera, Italy

That is the purpose of the ferrite, maintain wideband operation on the lines, IOW leave signal characteristics unchanged, while suppressing interference to outside the system. For twisted pair, the ferrite will capture and attenuate unbalanced current from the pair, and this includes pair-to-pair common mode currents required to charge the inter-pair coupling capacitance. This is another reason for using differential drivers, very low skew of duration very much less than the receiver response time, as well as low driver and termination impedance to reduce amplitude of net E-field induced currents.

On Mon, 01 Aug 2005 05:11:54 GMT, "Ban" wroth:

No effect only if the signal and return conductors are coupled together fairly well before the ferrite is applied and if both of them show the same degree of coupling to the ferrite after it's applied.

Ferrites applied to coax or twisted pairs are quite effective without affecting rise times. Ferrites applied to ribbon cables with random signal/ground conductor topologies are not guaranteed to preserve rise times.

Jim

It will do that but only by a relatively small fraction compared to other methods. There are two aspects to the twisted pair that eliminate B-field interference: 1) the current imbalance is minuscule so that net B-field around the pair is too; and 2) the alternating orientation of a victim pair relative to the net B-field from another pair results in alternating field induced voltages in series which cancel. This usually reduces the differential effects of pair-to-pair coupling to negligible levels. It is the common mode effect that causes problems, and this occurs only when the differential driver for a pair changes state, introducing a transient common mode due to slight differences in the propagation delays of the OUT-/OUT pair. The resulting dVcm/dt injects currents through all available coupling capacitances into victim circuits, and this current will be a common mode current, which means each line of a victim pair receives equal amounts of interference current. The common mode injection current does not cancel in the victim circuit and develops a common mode voltage at the receiver.