RS232 Programming circuit [Prop fried]
Jasper_M
Posts: 222
So, I built the RS232 programmer circuit described in Propeller data sheet. Well, almost that circuit. I used different transistors... and apparently the PNP transistor whose base the Propeller's TX pin is connected sinks pretty much current, even through a 1K resistor. I think that might be what caused the death of my prop.
Before I'll attempt again with the correct component values, I wanna make sure that the programmer is really meant for the RS232 levels on my computer, -10 - +10 volts. Should that be ok? Also, why doesn't /RESET have a pullup, and why is the prop TX transistor tied to DTR rather than ground through a resistor? Isn't this bit of a problem when the voltages between Tx and Dtr is 20 volts max?
Thanks, Jasper Mattsson
Before I'll attempt again with the correct component values, I wanna make sure that the programmer is really meant for the RS232 levels on my computer, -10 - +10 volts. Should that be ok? Also, why doesn't /RESET have a pullup, and why is the prop TX transistor tied to DTR rather than ground through a resistor? Isn't this bit of a problem when the voltages between Tx and Dtr is 20 volts max?
Thanks, Jasper Mattsson
Comments
/RESET doesn't have a pullup because it's effectively a pullup when BOE (brown out enable) is low.
The TX transistor is connected to DTR to provide the negative voltage required for RS232. It assumes that DTR will be active (at -V) during transmission.
The 4.7K and 1K resistors in the circuit should protect the Propeller from overvoltages. There are protective diodes to Vdd and Vss on all I/O pins and these will clamp any input voltages above 3.9V or below -0.6V as long as the current is limited to roughly 20mA which the resistors will do.