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Posts: 6

Sniffing a probably synchronous serial communication with a db9 connector?


Hello everyone!
I would like to get some help/advice about my task. I've got an RF transceiver card connected to a PC through a "serial" connection with a db25 cable. The card is controlled by codewords from the PC. I would like to make such a spy/sniffer cable, that can monitor the process of the codeword sending. The problem is the following. The pinout of the RF card's db25 serial interface (I attached a picture about it). If I'm right the pinouts means that the card uses a synchronous serial connection, because of the presence of clock. BUT preferably I would like to make such a spy cable, that has a db9 serial connector at the monitoring end. So one end is "injected" into the communication with the help of two db25 connectors, and one end is connected to the monitoring PC with a db9 connector on a serial port. Finally, make a labview vi to process the data through COM port. On the first base, I would only like to get the data traveling on the third pin (called Data Input from PC, but I think for this I'll need the Clock and Strobe too), and after I would like to try to monitor activity on all of the pins.
My questions are:
Is it even possible to read some synchronous serial data through VISA read, and if yes how??
How would you suggest to wire the connectors? The data <-> Rx is obvious, but is it possible to "foul" Labview and get the clock signal through the DSR or the CTS option???
Or is it much simpler if I use a db25 connector at the monitoring end too? But then how can I read data from LPT port?
I would appreciate any answer or advice!!
Thank you in advance,


Posts: 71

Re: Sniffing a probably synchronous serial communication with a db9 connector?

Dear Nagyo,


Your application sounds interesting Smiley Happy

The VISA driver uses a 8251 compatible chip in the PC. This chip can work fine with sync and async RS232 - USART also. So I think it's no problem to use it to obtain the transmitted data.

Right click on the VISA Write VI, and select Synchronous I/O mode > Synchronous mode




Source article is here:  http://digital.ni.com/public.nsf/allkb/ECCAC3C8B9A2A31186256F0B005EEEF7?OpenDocument


By sniffing the data, it is also important to do not disturb the communication between the devices. We should configure somehow a passive mode - don't wire the handshaking signals, and timing signals.

For synchronous data transmition, the DB9 connector doesn't have enough pins. The DB25 connector has the 15th 17th and 24th pins, wich are required in synchronous transmission.

I checked out the picture you sent, but for me it doesn't look like the standard DB25 conector's pinout. I found this website, which describes very well the cablings and pinouts for the different RS232 modes:



The LPT port can be also DB25 form, but I don't think that you can use it also as a Serial COM port. (Maybe some PCs supports it, I don't know) Probably you'll need a PC card with a DB25 COM port.


I hope, I could help you with some informations for your project.


Best Regards,



Proven Zealot
Posts: 9,719

Re: Sniffing a probably synchronous serial communication with a db9 connector?

As I mentioned in you previous thread this is not RS-232 There is a link to the manual in the other thread the manual will explain how to connect the eval board to the PC's Parallel port..  NI spy will let you monitor the port traffic.

Knight of NI
Posts: 18,065

Re: Sniffing a probably synchronous serial communication with a db9 connector?