Serial communication

Roadster

I have attached a schematic of the 2 transistor circuit for programing the propeller that has 2 additional parts that allows better compatibility with terminal programs including hyper terminal using the fullduplexserial object, it works by using the rx pin to charge a cap for the tx pin instead of using the DTR pin.

R Pankau

Does this circuit work? It's been a few months since your post and I don't see any other comments. I built a homebrew prop plug using their schematic and of course when the prop transmits it resets itself. I may rearrange my generic plug if I don't hear any negative feedback.

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necessity is a mother

Roadster

I see a error in the schematic, I will re-post the corrected one

Peter Jakacki

The RS-232 receivers don't actually need a negative voltage and there is nothing in the chip that does anything about it. There is no problem at all if you run R1 directly to ground. You don't need RS-232 compliance if you just want it to be RS-232 compatible. Even the old 1489 receivers only needed a voltage of 1.5 to 2V to turn-on and a voltage of around 0.8V t o turn-off. The negative voltage comes from the 60's 70's era when RS-232 was king and they ran lines for around the building with it, since it was single-ended it was susceptible to noise.

I wish they would come up with a new RS-232 "standard" which addresses RS-232 as it is used these days (very short runs and high-speed), but that might mean a loss of revenue for many chip manufacturers. In the meantime system manufacturers have to incorporate chips with charge pumps and +/- drive outputs just because they need to be compliant with an ancient standard. Large voltage swings limit the baud-rate because of the slow slew-rate of the drivers.

BTW, quite a few of my boards don't bother with any DTR reset feature as I simply power-up or reset the prop at the same time as I download. DTR reseting can be a problem in production systems and sometimes only the RX and TX are available anyway. Besides, other than development there is no need to be continually downloading new software so it's no hassle to do without.

*Peter*