Connecting COM port


I’m a newbie at this hardware stuff so I’m going to ask what most feel is a simple stuff but I don’t want to fry my Cobra.

I’m trying to connect the serial port on the back of a Crestron controller to COM1 of my Cobra. The Crestron controller’s serial port has pins 2,3, and 5 connected. I’m using 50 ft of Cat5e to connect to my Cobra. Can someone tell me how to connect this to my Cobra.

Remember I’m a complete newb and still getting my head around all this serial stuff :slight_smile:


You need to convert the levels with a 232 level converter IC. or you could use a usb to serial adapter or premade rs232 shield.

Note that utp is a very bad cable for rs232. Better use a shielded cable without twisted pairs.

I’m confused why I need the 232 level converter IC. I’ve connected the serial port on the back of the Crestron controller to other equipment such as home security systems and AV receivers without issues. Can you explain this?

Also what kind of generally available wire would work best for serial communication over moderate distances (less than 100 feet). I’ve used Cat5e between this processor to my AV reciever without problems but if there is something better I’ll sure listen.


Because the cobra is TTL level and the device you want to connect to it is RS232 level, you’ll burn out the pins if you connect them.

Any cable that isn’t UTP would work fine.Most stores sell a cable that has shielded non-twisted pairs that works well.

RS232 = -12V to +12V
FEZ pins are TTL = 0V to 5V

See the voltage difference? Not only that , the signals are reversed!!
true = 5V on FEZ but -12V on RS232!

Maybe confusing but this is a very very old standard and is commonly known among electronics engineers.

External rs232 ports (and I suppose on the crestron too) are using levels of up to -15v to +15v, while the com ports on the fez use TTL levels between 0 and 3v3 (and because the fez is 5v tolerant you can use a 5v converter).

I mostly use shielded liycy cable with .5mm wire section.

OK thats much clearer now and I see why I’ve had no issues with this relay in the listed in the link below.


I still don’t know what to connect where on the Cobra though.