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"Enable" UART pins as RS-232, what's needed?


#1

Hello,

I’ve been looking at what’s needed to “enable” the UART ports on the FEZ Panda as RS-232.

Not being an electrical engineer my first thought was to just wire the FEZ pins to a DB9 plug. I then realized the FEZ pins are 3.3V and RS-232 needs 5V.

I looked at the schematics for the EMX Development System and see that on the EMX board the UART pins are wired to a ST3222-TSSOP IC.
Is such an IC needed, what is it’s purpose?

Can that be done just using some capacitors, like here: http://www.watterott.com/de/RS232-Shifter-Board-Kit

Best regards
Mark


#2

Hello !

RS232 is not about +5V levels but -12 to +12 (-12 beeing logic "1"s).
You need and external circuit to convert the levels, such as the RS232 shield : http://www.ghielectronics.com/catalog/product/182

or such kind of adapter : http://www.robotshop.com/ca/droids-db9-serial-adapter-5.html

Cheers :D,
Nicolas


#3

Thanks for the quick reply Nicolas.

What is the difference between the Droids Serial Adapter you recommended (thx for the link, nice stuff on the site) and a “RS232 Shifter” like the link I originally sent, or this one http://www.robotshop.com/eu/sfe-db9-rs232-shifter-4.html

Best regards
Mark


#4

They are mostly similar, however the shifter is just a “trick”. It works with “most” rs232 cards, but not all. It may be ok for your tests at home or hobby apps, but should not be used for a profesional/industrial application.

Also, by its schematics, it is only “shifting” TXD and RXD. The droid is shifting all signals, if you would need them (DTR, CTS/RTS, etc).


#5

Thx for your help!


#6

Are you sure you want an RS-232 converter?
You may want to consider using a converter to USB - that way you can connect directly to a USB port.
Something like: http://www.sparkfun.com/products/718


#7

Well, it seems like a waste to use the USB for that. Nevertheless the board has four UARTs, right.
Is there any advantage to use the USB to serial converter?


#8

No. I thought you wanted to connect to a PC. In that case the connection with USB is easier and the board is cheaper than the shifter you linked to. And most PCs don’t have serial ports anymore.
If you’re using the RS-232 for other connections, then there’s no sense in USB. Use the shifter.