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4V level UART polarity (inverted RS232) input on a Panda


Would you expect that a device with a programming port described as “4V level UART polarity (inverted RS232) input” would work with a Panda COM port? Seems like there is between 2 and 3 volts on the line depending on how many other pins are in use at the time. Has anyone dealt with something like this and used something special to keep it at 4V exactly? I have a radio module and I can get it to transmit a modulated signal but I can’t get any serial commands to work at 2400/8/N/1.

Full manual is here -


FEZ is 5V tolerant so 4V won’t cause harm but I can’t guarantee it will work. Will probably work though.


This is from the Panda to the radio module (sending commands from netmf to it), not as an input to the Panda. I’m just curious if anyone has ever run into the voltage not being exactly what something needs from the COM port and had to rig up something to get it to work right, a transistor and voltage regulator I’m guessing. The Panda does do inverted RS232 though correct?


You did notice that the pin names are reversed, right?

RX on the module is the output, TX is the input… :slight_smile:

Edit: Sorry, you are using PGM in…


Yup, COM1 out on the Panda to PGMIn on the module so I’m pretty sure that is correct.


I measured the voltage on the COM port of the Panda and it is 3.4V. I measured it on an Arduino and it is closer to 5V which surprised me since I thought the attempt was to keep it compatible with an Arduino form factor. Has anyone else ever had an issue with the COM port voltage level causing an issue with a device?


Fez is 3.3v so what you see is normal. Either way 3.3v or 5v are both TTL compatible


it is compatible with the arduino form factor. Not the voltage level.

netmf boards are always 3v3 not 5v (are there any exceptions? cant think of any). You’ll never see that one specific feature, RS232, be the same. Most TTL/CMOS logic will work with 3v3 but this specific rs232 scenario might not be “compatible” but where’s the data sheet, it should talk about acceptable levels.


Actually, every single modern processor is 3.3V or less…5V is usually seen on ancient 8-bit micros :slight_smile:


I forgot to mention, I took a look at the datasheet and it’s, well, crap. Doesn’t tell you enough to get this to work. Unclear whether this is true TTL or true RS232 with a voltage constraint. If you haven’t tried, have you considered getting a MAX232 (or similar, get one without the need for external capacitors if you can) and try that? I’d watch the voltage level implications (frying your wireless module isn’t something you want, right?) but it seems totally unclear from the information at hand how best to proceed/assist you. Might be worth going to the vendor…


I have, according to them 3.3V should work. I did actually try what you suggested and it didn’t work so they are going to ship me a known working unit and we’ll see from there.