We have a task in hand to sniff a RS232 protocol in a certain industrial automation setting (we have been asked to do this legally by the manufacturing company itself). We are planning to send a technician for this task (as it is overseas) and provide him with a hardware/software serial port sniffer/analyzer so that he can send us back logs with timestamps and protocol definitions and inturn our software team can define the protocol looking at those logs.

Does anyone have any ideas for a hardware/software serial analyzer solution that can log data and timestamps that can satisfy our task?

  • \$\begingroup\$ I'm afraid shopping / buying recommendations are off topic per the faq. \$\endgroup\$
    – user17592
    Mar 30, 2013 at 15:45
  • \$\begingroup\$ If you can install software and configure one side or the other perhaps com0com.cvs.sourceforge.net/viewvc/com0com/hub4com/… \$\endgroup\$
    – kenny
    Mar 30, 2013 at 16:35

2 Answers 2


There are commercial solutions but here's the schematic and photo of a little DIY system I made a number of years back to handle the hardware side things, from left to right passes through all signals between the system being analyzed and the top and bottom you can connect to a pair of RS-232 ports on a PC or laptop. Port A at the top for example will show data received by port A on the left:

RS-232 Analyzer circuit

RS-232 Analyzer

To explain the above further here is the pinout of a male DB-9 connector taken from Quatech QSC-100D documentation which is the DTE connector you'd typically find on a PC:

Male DB-9 Pinouts

So the grounds are connected together between all connectors and pin 2 is the receive pin allowing data to be received on a PC at the top and bottom. If you were only interested in data flowing in one direction one of those connectors could be omitted, but it may make send to include both in case you require it in the future. The left-most connector is male and the remainder are female DB-9 connectors.

For software if running Windows I'd recommend RealTerm which has a nice array of display options including hexadecimal and it supports logging including timestamps.

  • 3
    \$\begingroup\$ I used to work for a company that made such break-out boxes some 20 years ago. We did it exactly the same, just put it in a neat housing so the customer couldn't see the sloppy soldering :-) \$\endgroup\$
    – Tonny
    Jun 20, 2014 at 13:07

I have done a lot of this type of sniffing. Most often you only want to connect two pins on the Dsub 9 connector to your laptop, Pin 5 (gnd) and Pin 2 (receive). If you are using an older Windows XP laptop you can use HyperTerminal. If you are using a newer Windows 7 (or Vista) operating system you can copy the HyperTerminal.exe from an older operating system.

There are a couple of different Emulation settings and it would help if you knew a little about the protocol you are sniffing as it may require a particular setting (ANSI,TTY,VT100).


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