RS-232 sniffer with a twist

Hi
I am trying to build a simple comm spy cable / circuit which will enable me to monitor comm between DCE and DTE.
Now I have built one and it works absolutely ok between 2 PCs but when I want to use it to monitor communication between an embedded device and another embedded device it just shows me garbage/ diff data then expected when I put my serial sniffers probe on the Tx pin of the device. In theory I think it should work if I take Tx from the embedded device and gives it to the Rx of the PC port with Gnd attached. but It gives me the bad characters which is not the actual data I want to see.
what should I do here,, do I need to build some special cable or circuit for this..
specs of Embedded Device A:
basically it is a custom built device built for updating EEPROMS/ Flahes of other embedded devices we have.it is a FTDI 2232C based device takes instructions from pc / host software via USB and then uses 74125 Tri state buffer in between and finally sends out the data thru converted serial .Now I dont know why 125 is used here as I dont think(based on my limited knowledge, correct me if I am wrong) there is a need, all the conversion has already been done by 2232C FTDI I think.

specs of embedded device B:
It can be any device which needs to be programmed ,

Now what should I do to sniff the serial comm b/w both of the devices.
please help
thanks

Interesting...how very clandestine...Is this thread related to the other thread you posted regarding the fingerprint verification device made by sony? I believe you said the sony device works on rs232 also right? Maybe the data from the fingerprint verification device (that you are trying to bypass?) is encrypted, then decrypted via some software. Just a thought.

Guess

No they both r different mate ...

fiu300 proj is different and sniffer is different..
I just need some simple jumpstart...rest I'll do
I am not asking anyone to give full step by step instructions ...
but just a few hints frompros out there will do

Is it at RS232 or TTL levels?, bear in mind that they are inverted from each other!.

Levels

Thank you very much for the reply mate
I think the device must be at RS232 level as it communicates with the mobile phones on UART interface.
and secondly the device itself is FTDI 2232C based, and I guess it is at RS232 voltage level ...
Now a very basic question,,, is my idea of making a sniffer right ??
Should it work the way I thought ...
could you please suggest me some ideas of doing this.
cheers

I am confused.

You have built a cable for monitoring the RS232 comms between twp PCs, and that cable works ok. So far so good.

You are now wanting to use that same cable to monitor the comms between some other devices, one of which appears to be a mobile phone, if I understand correctly.

You speak of an FTDI 2232C, this is a UART which produces USB.

USB is not the same as RS232, the signal levels are different and the protocol is different, so your cable will not work.

JimB

There's a tons of PC software who already do the job. Somes are free. A simple google search may give you some hints.

I use those of HHD Soft
www.hhdsoftware.com

More details

I am sorry If I have not been successfull in explaining the thing clearly.
ok let me try again
a. I made a cable with simple one diode/ resistor for monitoring half duplex comm going on b/w two PCs via serial port , and on the spy pc i run HHD soft or / stealth monitor , It work perfectly ok.
b. Now I have a device which is not more than a USB to RS232C converter / 2232c is used which gives me RS232 port at the output. Now this device connects to the host PC and takes commands from the host software running on the PC. then I think the commands are sent out to the mobile phone via rs232 provided by the 2232C.
c. Now what If I put the Rx of my serial port sniffer from spy pc on the Tx of the device then it should display the data being sent to the phone...
atleast this is what I was able to see if I use the cable b/w two PCs and was able to capture the data using HHD or any serial port monitor.
d. The level at the output of the device should must be RS232 , it must've not been TTL as the mobile phone I think talk on UART and it should be RS232.(I dont know I may be wrong here .,.correct me if I am wrong)
e. Now my question is that is my theory ok,,, what should I do to make it work / practical.
f. should I use Max232 b/w the Tx being monitored and my spy Pcs serial port to make the levels same ... i.e RS232 at both ends.. but I dought if this is required I am already sure that both are at rs232.
g. one thing i noticed, that my serial port monitor automatically detects the baud rate when I sniff the data b/w two Pcs , but doesnt do it when I try to sniff the Tx of the device. any clue ...
any help would be appriciated.
please bear with me , I am just a beginner


thanks

Can you drop a picture of what you have? That will help. It sounds like you are on the right track. If we see what you are doing it will help.

Does your PC work with the device "without" your Rs232 tap? Then you drop the tap in and it does not work?

As Nigel stated. If it is TTL levels, it will be inverted.. As you say embedded device.

I picture will help out a lot.

The load may be a problem also.

thank you

Thankyou very much to all of the Pros who helped me ...
The problem was with the level conversion ...
my bad,, I didnt think of it earlier ..The hints which u gave me actually gave the lead to add max232 in between ...
thanks for your help and hints u gave me .. I put a MAX232 in between and everything is now fine...
thank you very much once again ....