How to listen to Digital data in a cable

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fireball003

Joined: 28 Oct 2005
Posts: 194
Helped: 5

05 Nov 2009 1:52 How to listen to Digital data in a cable

Hi,
I will have to collect data from a digital trasmitting cable whose protocol is known and data is not encrypted. How to listen to data without hurting the regular data flow (electrically without voltage or current drop) ?

Any idea?

FYI, this is not any illigal hacking project. This is part of a system debugger project.

Please suggest me how to do this.

Thanks in advance.

betwixt

Joined: 04 Jul 2009
Posts: 384
Helped: 62
Location: Wales, UK

05 Nov 2009 10:53 Re: How to listen to Digital data in a cable

You need to tell us what kind of data it is and what type of cable is used. There are many different ways to send data down many type of cable and the technology to tap into them can be very different.

For example, I carry slow speed RS232 down CAT-5 cable along side 450MHz QPSK in co-ax, the two are entirely incompatible systems and require different types of receiver.

Brian.

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fireball003

Joined: 28 Oct 2005
Posts: 194
Helped: 5

05 Nov 2009 19:09 How to listen to Digital data in a cable

Hi,
The cable is twisted pair cable (2 wire). The data is Manchester coded. Data speed is 2.5 MB/s. We know the protocol of the data. So, I think we can extract the data from the RAW data. But collecting data is the problem so far.

In this network only "master" saves the data and controls the data flow. And there can not be 2 master in one network. So, we can not set our device as master to collect the data from the other master.

Thank you.

banjo

Joined: 24 Dec 2005
Posts: 644
Helped: 118

05 Nov 2009 19:56 How to listen to Digital data in a cable

The first thing to try is to get the same receiver IC as used on the slave end and parallel it across the twisted pair cable. This sort of SPY connection often works. To minimize the effects of reflections, it is best to make the parallel tap at the slave or receiving end.
When adding this extra SPY chip, you do not want to terminate the line, leave any termination resistors at the existing receiver only.

If that does not work, then the next trick is to open up the receiver box and tap into the digital signal right after it converted from twisted pair to a digital format. Hopefully, there is a dedicated receiver IC and the tap point is easy to get to.

Once you are capturing the data, you need to feed it into a logic analyzer for analysis. At 2.5MB/s, the data is too fast to just feed into a PC serial port. Depending on the logic analyzer's software package, you can use it to do the Manchester decoding and then display the data in a hex format.