Ethernet to RS232? (What's in the data?)

donl517

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donl517
Join Date
Dec 2005
Location
Michigan
Posts
127
I have a customer with a barcode printer connected to an ethernet network cable. They want to interrupt the network cable and split the signal to two RS232 devices. I can get the hardware to do that, however I don't know what format the data coming down the ethernet cable is in. I have no idea if it is even something I can work with as an ASCII string.

Does anyone know of a software package that would monitor the ethernet signal and let me see what I'm dealing with? (Without having to purchase the hardware needed to try this.)

Thanks,
Don
 
Do you even know if the protocol is "ethernet", or is the cable just a std CAT5 cable with RJ45 ends?
There are some protocol software tools that let you "Snoop" into the data exchange. Take a look on the net....
 
Just do a google of "Network Sniffer" and you'll find some trial packages. I use "SoftPerfect Network Protocol Analyzer", which works great and is reasonably priced if you decide to purchase it.
 
Ethereal at www.ethereal.com, they have Linux and Windows versions and you will like the price because it is open source. Ah just tried the web site and the program has now reverted to its original name of wireshark at www.wireshark.com. Here is the wikipedia page for it. Don't forget that you will have to be on the same ethernet segment as the bar code scanner, it won't work if the devices connect via an ethernet switch.

Bryan

[edit] cross post with Mart B
 
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Wireshark is a very nice tool.

But remember that to be able to 'sniff' the data being exchanged between 2 other devices, you will need an Ethernet hub (very hard to find these days) or a managed switch with port mirroring capability.
 
I doubt that will work for several reasons. I am sure both devices are communicating some protocol, therefore you will probably only see the first exchange and it wouldn't help you.
The second possible issue is that if the communications are over TCP, then your PC would have to be listening on a certain Port number and you would have to write an application for this.

Another possiblity is if you have 2 Network cards in your PC, sometimes they will let you bridge them together and your PC will act sort of like a hub and Wireshark will let you sniff the data.

I think this trick is documented in the Wireshark wikepedia or help somewhere.
 
Are you sure? I'm running it here on my network and I'm seeing, I think anyway, all the traffic on my network. I'm running through a Linksys router. I'm seeing activity from the other two computers & the network printer on my network.

Thanks,
Don
 
Yea, I'm sure. If you search through the Wireshark stuff, you'll see a lot of stuff on it.
Ethernet switches are designed to route packets to the physical port of where the devices are located.

See this link: http://wiki.wireshark.org/CaptureSetup/Ethernet

If you scroll down to "Switched Ethernet" and read that section, this explains the issue I am talking about.

You should only be seeing stuff going back and forth to your PC or other broadcast-type packets, such as ARP requests, etc...

You can certainly try to make your PC mimic the Printer and see if you can capture the data. Doesn't hurt to try but I don't think it will work.
 
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Yep I pretty well had it figured out. I tried to send a print job to my network printer from my laptop while monitoring it with my desktop. Never saw a thing. I'll do some research on the website.

Thanks,
Don
 
For these cases where I need to sniff through a cheap switch (i.e. not a managed switch with no port mirroring or monitoring functions) I set up a bridge with two ethernet cards. This goes around the switch issues all together. I use two USB ethernet adapters and Windows or Linux both have bridging software either built-in or available. A little more complex, but if you have to get to the data, this will always work. If your using XP, if you right click on My Network Places and select the last option - Properties I think, all the network connections will show. If you right click on them, there is usually an option for 'Add to Bridge'. Vista has 'Bridge Connections'. Linux requires a kernel configuration so is a little more complex if you have to compile your own.
 

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