connection to ground

hello guys another noobish question,

if im trying to get serial data of a magnetic card reader that has a rs232 chip what i would like to do is get the serial data before it goes into the chip before it changes to rs232, can i just connect the tx rx ttl level side to the tx rx on the arduino? do i also have to connect a common ground?

thanks neil

No, the signal must go thru an RS232 chip or equivalent, otherwise the +/-10V coming out of the card reader will damage your arduino chip.

Yes, grounds must be connected.

Here's an inexpensive way to get there:

http://store.nkcelectronics.com/rs232-to-ttl-3v--55v-convert232356.html

or http://store.nkcelectronics.com/RS232-to-TTL-converter-board-33V-to-5V-Wrong-DB9-male-adapter_p_259.html if you're a little bit handy.

it aleady has a rs232 driver chip a sipex 3220ec. i thought i could tap into pins 9 and 11 which is the low level logic into the driver chip

You will have an electrical conflict there. You coul pick the card reader's TX output to go to the Arduino Rx input. The arduino's Tx output will fight with the RS232 chip output over driving the Rx input into the card reader. The MAX232 doesn't have a way to disable it's outputs, short of you desoldering a pin and lifting it up.

thanks for the reply crossroads do i still have to share a common ground?. as in the only wire would be tx from card reader? thanks for your help

Yes, still need the ground connection.

thanks for your time and help crossroads