Voh [really, Voh(min)] is the minimum output voltage a driver will put out for a "high".

Vih [really, Vih(min)] is the minimum input voltage a driver is assured to interpret the voltage as a "high".

Voh - Vih is your margin. If the margin is positive, all will work well in a system provided the additive noise is less than the margin.

Traditionally, CMOS Vih is 0.7*VCC volts, and CMOS Voh is 0.8*VCC volts. TTL Voh is ~2.2 volts, and TTL Vih is ~2.0 volts.

There is no margin if you drive a CMOS input with a TTL output.

CMOS Vih~=0.8*5.0 =4 volts.
TTL Voh~= 2.4 volts.

No margin, and the receiver will likely not correctly interpret "high" signals, and may in fact behave very badly.

There are other buffers that take TTL level inputs, and produce CMOS level outputs. All servo companies should use such buffers, at least to 6.0 volts. (I haven't looked to see if they exist for up to 8.4 volts).

So, ideally, Vih will be 2.2 volts (TTL level), and Voh will be .8*Vcc (CMOS levels). Then, say, at 8.4 volts, and with a receiver driving TTL level Voh, you'd have about 4.5 volts of margin for a "high", that is, 0.8*8.4 - 2.2 = 6.72-2.2=4.52 volts.

If a servo has a CMOS input buffer, and the servo voltage is 6 volts, say, then Vih=0.7*6 (CMOS levels)=4.2 volts. Still not enough margin even for such a servo, if it exists, powered at 6 Volts.

If a receiver is driving it's output at 3.5 volts, there is negative margin for noise. Then you would need a buffer with CMOS outputs and TTL level inputs, which would drive Voh=0.8*6=4.8, leaving 0.6 volts of margin.

Now, I don't know which receivers drive TTL level Voh, or which servos take CMOS level Vih, but if this happens, shame on both the servo makers and the receiver makers for not putting this sort of data in their specification sheets. (But then again, that provides a hole for a niche product).

It's a little more complicated because I mostly considered noise margin for signal high. You also have to consider the noise margin for a "low".

Anyway, hope this helps homer

Exactly!! Actually, that's all Greek to me....but I copied it and am sending it to my RC buds.....just to mess with'em

I'm looking for a younger wife

You'd be better off with a richer wife. Even Homer understands what you were saying though.

Did I misunderstand the video?

Oh well, keeping my wife. Looks and acts young, good mother, made a vow. She's already more than I deserve.

I don't know a great deal about servos, but shouldn't they do something a bit more clever rather than assuming certain Vih and Vil thresholds? Ie they ought to be looking at the average of the input signal and then using this with some hysteresis to recover the very important pulse width information. Right?

Blue Iceman, they could use a schmitt trigger or similar buffer with hysteresis to accomplish what you're talking about. They probably do. I've never examined the logic chips in my digital servos.

Between Vih, Vil, with or without hysteresis--input buffers will have a linear region. It's the nature of single ended digital logic buffers. Things are much worse in that the same power that is being used to drive the servo motor is also used to power the logic in the servo. This is also robbing from the noise margin. Between ViL and ViH, a logic buffer functions as a high gain amplifier. Since it is not intended to operate in this region, the exact point where the output goes from low to high can change from chip to chip and in different voltage and temperature environments. If Voh + noise (where noise can be + or -), causes the voltage at an input buffer to go into that linear region, bad things ensue.

I think servos should be differential signaling over impedance controlled twisted pair, with embedded clock. This would have much better noise immunity. Ideally, there would be two heavy gauge wires for servo motor power, two lighter wires for logic power, and two wires for differential signalling. That is six wires. Get rid of PWM, go PCM with error correction coding, and maybe even TDM so that the servos can act as repeaters and send buffered signals to the next servo in the line.

Most people would probably reject it, it would require a different way of wiring up airplanes, new receivers, cooperation among radio and servo manufacturers, and probably would only have payback in larger scale RC aircraft (very small market?)

Lone Wolf you might have identified the new product for Smartfly. All that you describe could be done a mothercard that would accept current style receivers (which would look a lot like current Smartfly products) and a little daughtercard that would go with each servo. It wouldn't be as clean as being integrated into the servo, but it would address noise immunity and power distribution. Since you are already doing format conversion you could throw in servo balancing - which would then be available for as many servos per channel as the micro had memory for, and could be done for every channel.

While there could be endless arguements about the optimum wiring harness (6 wires, 4 wires, 4 plus optics and so on) the basic concept might be killer, and once the wiring was agreed might eventually take over for OEM servos.

Hi,
Problem with this is cost. Doing any daughtercards or similar ideas really drives the cost up because of the connector costs and costs of assembling secondary board. People already complain about my cost and it does not make sense to do a board like this in very small volume just for those people that really care about getting optimal performace from their systems. I really wish they did care more but generally its the attitude "If I don't crash then my system is fine".

Originally Posted by CatalinaWOW

Lone Wolf you might have identified the new product for Smartfly. All that you describe could be done a mothercard that would accept current style receivers (which would look a lot like current Smartfly products) and a little daughtercard that would go with each servo. It wouldn't be as clean as being integrated into the servo, but it would address noise immunity and power distribution. Since you are already doing format conversion you could throw in servo balancing - which would then be available for as many servos per channel as the micro had memory for, and could be done for every channel.

While there could be endless arguements about the optimum wiring harness (6 wires, 4 wires, 4 plus optics and so on) the basic concept might be killer, and once the wiring was agreed might eventually take over for OEM servos.

I really enjoy my Smart Fly hardware
From the boards and regulators to the HD and pin switches.
I just purchased a Turbo reg from SF and a brand new Comp 12 from a friend
As I mentioned earlier..I don't assemble a 50cc or larger plane without having a smart fly board, switches and regulators (if needed) installed in the plane.

Having said that, my next regulator and switch purchase will have to be from BoomaRc.com to try out the Intellireg HD and a couple of their intelligent switches with lcd readouts.

Having the LCD readout of battery voltage, mAh usage and such via the switches and regulators is just too nice of a feature to not have handy, considering the price that Booma has these at.

I'm hoping that SF has these features in the works..because I'd prefer to support SF

Originally Posted by rrritchey

for those people that really care about getting optimal performace from their systems. I really wish they did care more but generally its the attitude "If I don't crash then my system is fine".

lol. Man ain't that the truth.

A friend of mine just bought a motherboard with a dual core CPU and an embedded GPU and BIOS, with, connectors for $60. Motherboards often come cheaper than I can buy the connectors for.

I am about to give up myself. I don't know how I can compete with that in my line of work. I don't see how they ever recover the cost of development. I am condemned to integrating "IP" from others, and fixing half of the "tried and true" IP I have to use.

Maybe, get rid of the connectors. Let the end user solder wires to plated through holes. Have daughter cards that solder right onto the motherboard using castelations. (I "invented" this about 11 years ago, out of necessity due to a pinout mixup for an IC on a very expensive PCB--I should have patented it--they patent anything these days...but many other people invented it later, independently I believe, and for all I know, it was invented before I did it.) Basically, pads on the motherboard, and half plated through holes on the outside of the daughtercard, or similar. This is how people are doing it today.

Maybe you offshore the daughtercards. I have friends who have done this and gotten truly amazing cost and very good quality. I know all of the objections, but it is worth a thought.

The bright side is that if the market is as small as you believe, it won't be pirated.

I keep throwing ideas out there. How about lead connector for wings such six or nine wires in( two or three leads in), six, or nine out, (two or three leads out) for the servos. One connector. Seeing some guys converting computer connectors, for RC use. RUTNBUC

Hi,
I did have some special 8-pin Multiplex connectors made up I was planing on using on special boards. I just do not have them on the site. They will support up to 4 servos in the wing. Contacts are 10A each. Also have a panel mount version.

Originally Posted by wylieruneyjr

I keep throwing ideas out there. How about lead connector for wings such six or nine wires in( two or three leads in), six, or nine out, (two or three leads out) for the servos. One connector. Seeing some guys converting computer connectors, for RC use. RUTNBUC