DB25 to RS232 pin wiring

hi all, i have a video switcher im connecting via rs232. attached is a picture of the pin diagram, apart from the obvious rx,tx and ground. how can i use the other pins to allow for better connection (common=async?)

It appears that "Common" is what the "Alarm (NO)" and "Alarm (NC)" connect to. Assuming that's true, if you want to read the state of the switcher's alarm output, connect the "Common" pin to ground, and the "Alarm (NO)" to an arduino pin with a pull-up on it. Then that pin will go low when an alarm is detected.

Ran

thanks ran, i red that i can get rs232 communication with "just" rt,xt and ground but with no control flow and it may not work. i will test and see. thanks.

Wiring up something nice and simple, for instance a plain old "dumb terminal", is just a matter of connecting Tx, Rx and Ground, right?

Usually Not. While the normal PC hardware might well run with just Tx, Rx and Ground connected, most driver software will wait forever for one of the handshaking lines to go to the correct level. Depending on the signal state it might sometimes work, other times it might not. The reliable solution is to loop back the handshake lines if they are not used.

http://www.airborn.com.au/rs232.html

Yes, there are some PC programs that don't work correctly without handshaking, but the Arduino doesn't use it.

The one thing you might need to worry about is that, if the video switcher is sending really long messages, and the Arduino doesn't fetch the data out of its receive buffer fast enough, you might lose some data. Since the maker didn't set up the interface to use hardware handshaking, that's not likely to be the case.

Don't be surprised if the Tx and Rx on the video switcher are the opposite of the ones on the Arduino: it's pretty common for equipment makers to arbitrarily choose naming based on what they think will make sense to their customers. Which might or might not be what makes sense to you. :)

Ran

it's pretty common for equipment makers to arbitrarily choose naming based on what they think will make sense to their customers.

Its not quite as random as you think. It depends on whether the equipment is classed as DTE or CTE. Data Terminal Equipment (DTE) or Data Communications Equipment (DCE);

See the pin out at:-

http://en.wikipedia.org/wiki/RS232

thanks, it worked tx-tx, rx-rx, ground-ground