Damaged 1 Prop pin (Tx), could DIRA~~ avoid it?

MacTuxLin

Hi all,

I have successfully damaged a prop pin which was connected to a servo motor during a power failure at the servo motor side. I thought that only the Rx pin on the prop needs protecting via a 10k resistor but it was my Tx pin that was damaged.

May I know if placing DIRA[TxPin#]~~ could avoid this problem or its better that I place a diode?

I made another board that has 1 LED per available prop pins & have a simple problem run the LED & check the prop pins. That where I realised that Tx pin was damaged.

JonnyMac

All pins are the electrically the same, the TX and RX pins happen to be used by the Propeller for programming. It's not a good idea to connect a servo directly to any pin, though a 10K resistor seems a bit excessive (other than for current protection for a servo failure that is very rare).

I'm working on a circuit now that uses 100 ohms in series and a 0.01uF cap to ground to filter motor noise from the servo.

kwinn

The dira or a diode may have protected the pin, but it depends on the voltage coming back from the servo. Even a 10K (or higher) resistor is no guarantee of safety. If the highest voltage connected to a peripheral circuit can drive more than 0.5mA into a prop pin, then that pin (or the whole prop chip) can be damaged.

The closest thing to a guarantee would be to use an opto isolator with a 7.5KV breakdown voltage.

Murphy's law always applies.

MacTuxLin

Thank you JonnyMac. Sorry, I didn't mean "the" prop's Tx pin. I actually assigned Prop's pin15 as Tx to servo motor's Rx & Prop pin14 as Rx to servo motor's Tx. When the power source that's feeding to the servo motors failed, it never cross my mind that it would affect prop's pin until (after spending 1 whole day replacing every items on my prop board including the E²PROM except prop itself ) I actually replace another 40-pin Prop chip. Thank God I had 10pcs on-hand!

When you mentioned 100ohmns in series, do you mean Prop Tx-assigned pin -> 100Ω -> servo motor's Rx and same for Prop's Rx -> 100Ω -> servo motor's Tx and a 0.01uF at the servo's Vcc/Gnd?


Thank you Kwinn. I think you've help me see the problem. The main adapter's current to the servos could be too high. I'd better check again. Also, could you please recommend a brand/part# opto isolator?

Mike Green

A 4.7K series resistor should protect a Prop I/O pin from a servo failure for pretty much any servo operating off up to 7.5V. As mentioned, it does this by limiting the potential current through the substrate protective diodes for the I/O pin to less than 0.5mA.

The major complication with series resistors is that the resistance along with possible capacitance in the load (servo motor) may form an RC network that will slow down the signal transitions and limit the speed at which the I/O can function. In the case of servos, this is pretty slow (10s of us) and you'd need a lot of capacitance between the servo control line and ground (say 5uF) for a 4.7K resistor to affect anything significantly.

MacTuxLin

Thank you Mike. So, in series as in this? (attached picture). I though I've read somewhere that I should always have a pull-down on the Prop pins especially if it is a Rx. Appreciate your advice.

Thank you.