Hi,

I have a board with with a microcontroller. This board also has the serial port wired up and is provided with a female db9 connector. I want to connect this to another board that i will use for data transmission. The data transmission board has a female db9 connector too, I tried the following but in vain
1. use a male gender changer between the two boards - no interrupt on the controller side for the data that is sent
2. a null modem cable with female on one end and male on the other, i attached the gender changer to the female end and there is no communication either.

I have been searching the internet for cables that connect two DCE(modem) devices but havent found one.

Please let me know your suggestions.

TIA,
Bob

 

Here's some help:
http://www.beyondlogic.org/serial/serial.htm
It's not always as easy as a simple null modem adapter though.

Be certain that your port speeds are the same, and you have the same data, stop and parity bits.

Are you certain that they are both DCE's?

 

Thanks sir, I was expecting your reply.So I am not entirely sure how i can conclude if both the devices are DCE. The one indirect way i used was to connect the boards individually to a PC with a straight cable. Since PC is DTE the other device should be DCE as communication is fine. IS there any other way to verify that the devices are indeed DCEs?

 

Well, if you could communicate with both of them individually via a PC com port using a straight cable, then they are indeed DCE's.

 

Any other ways to debug this ?

 

Here's another page:
http://www.bb-elec.com/tech_articles/FAQ_rs232_connections_work.asp

You can tell for certain by using a tester plug. If you don't have one of these, you can make some using some LEDs, resistors, and D-sub 9-pin connectors. They want an arm and a leg for theirs.

If you compare the null modem wiring between the two websites, you'll see that they differ. Sometimes the devices will be very particular about which kind of null modem adapter they want. Sometime they'll work with just the TX and RX lines crossed, and their own CTS/RTS/etc lines talking to themselves, as in the 1st websites' wiring:

This 2nd website shows the wiring fully cross-connected: