when to isolate
I'm curently building a robotic device designed around the propellar chip. The device has accessory pushpull conectors on the case for future serial accessory use. Is there somthing I should/could do to protect these io pins? is it ok to have them conected to the outside of the case without opto isolation? when is it importient to isolate the pins on the propellar?
Thanks,
Owen
Thanks,
Owen
Comments
The choice of whether to use opto-isolation also depends on your confidence in your building. There are a lot of off the shelf modules with opto-isolation that strictly could live without them but as the vendor cannot guarentee that the user knows his stuff it can be a much safer belt and braces approach.
For what I'm interested in I'll probably use inverter chips as buffers to couple with my 5v systems and only use isolation in the case of noise problems. I'll also be interested to hear what others have to say on the subject though, good question.
Graham
There is a sticky note about I/O interfacing.
If you include a resistor in series with the pin the internal parasitic diodes will protect the pins to some extent.
Depending on the wiring length etc you could add external protection circuitry like transorb diodes etc then optos for longer wiring.
If it is a mobile robot and small then just the resistors should be enough, high voltage servos and fixed robot then add as much as you can. Welding robot then optos, ferrites, inductors, commonmode chokes and a bunch of stuff.
Gavin
For instance, in the Hydra the game card anticipates this problem and will tolerate 'hot plugging' with no problem at all.
But both keyboards and mice [noparse][[/noparse]which can use 250ma] cause problems with hot plugging ON ANY DEVICE and may damage themselves and other things. Documentation consistently warns against removing or installing these devices after a computer is turned on.
I have yet to see anyone actually propose opto-isolators on keyboard or mouse interfaces, but I am considering them as a more durible solution and they actually can allow one to jump from 3.3 to 5 volts at the same time.
Of course there is an added cost due to the added part count. So if it isn't a problem, isolation can be omitted.
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Post Edited (Kramer) : 12/17/2006 8:36:42 AM GMT
Owen
Also parallax is in the process of creating a data-sheet for the propellar. judging from some of the tests they've talked about on the forums, it'll be hard to fry a Prop pin hooked strait to a connector. Add a current limiting resistor and i'm nothing short of pre-meditated destruction is likely to kill a pin or the prop.
I think a larger problem with hot-plugging is on the software end. how do you recover from an unplugged periferial? how is a newly plugged periferial identified and configured? One of the key reasons PS2 stuff is not hot-pluggable is that PS2 stuff is only recognised during the boot process.
My 2 Cents
Marty
From what I have seen.. its a chip-based micro-transformer IC solution combined with high speed CMOS.. interesting..
if you cannot use the link.. just go to www.analog.com .. then Interface ... then Isolators.... I haven't used this product... but it claims to be much more energy efficient than the opto-couplers alternative... and the gates are bidirectional.. which takes care of the one way direction nature of the opto-couplers (good point raised by Lawson)
Even better.... you can order samples of several versions of this product through their page.. (this product in particular comes in SOIC package only)
I hope this helps!!!
"Propeller ... Unleash the REVOLUTIONs"
First is that isolation requires separation of power supplies into two different supplies.
And secondly, hot swaping may still damage the keyboard or the mouse or·other device·- reguardless of using opto-isolation of the Propeller.
Lawson mentions an idea which is basically a 'switching scheme' that is quite sound. And yes, the SXes and the Propellers are 'hard to fry' when compared to Atmel devices. Generally, they sink and source 150% or more than the competative devices.
Personally, I would prefer a case with a power interlock. If I wanted to reconfigure, the power is shut down whenever I open the case. It certainly is a less complex solution, but not one that is always possilble.
I really don't know if using curent limiting resistors will really do much in all situation.· After all, the assumption with them is that the inputs are roughly less than 5.5volts.· If you plug in 12 or 24 volts, there isn't a sacrificial chip inbetween and the resistor's value isn't enough.· Even using a simple inverter or driver chip can protect the Propeller better.· The chip gets destroyed, but not the Micro.· Lost at 74HC06 and 74HC07
Worry about ground loops and noise if you have a real problem.· These are hard to anticipate and often too much is added to a design.· Then it becomes are greater burden to analyize the true nature of the problem.· In other words, keep it simple.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
Post Edited (Kramer) : 12/18/2006 10:01:16 AM GMT
I have used this one before from maxim. They have others as well.
basically you set the cutoff voltage and it fault protects your circuit.
plus you can switch the lines or just tie the lines that do the switching to vdd or vss if you don't need them.
they offer some with 75 Ohms and some with 100 ohms and thats all the effect it places on your signal.
And you can get them in dip's.
anywho, have fun!
In cutoff the device only lets nanoamps through to the protected side of the chip.
I suspect the switching ability could be used to dicconnect the periphial's signal lines prior to removeing the device.