Electronic – RS 232 serial sniffing

embeddedlogic analyzerrs232serial

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?

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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.

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Electronic – the maximum acceptable baud rate error for RS-232 serial communication

Feel free to edit in more values. These are the ones I'm using now.

Atmel ATmega UART:

5 bits, normal speed : ±3.0%
5 bits, double speed : ±2.5%
10 bits, normal speed: ±1.5%
10 bits, double speed: ±1.0%

NXP LPC111x (Cortex-M0) UART: 1.1% (no table)

Microchip PIC 18F2XK20/4XK20: not defined. (wtf.)

As the venerable Mr. Horowitz & Hill put it:

By resynchronizing on the START and STOP bits of each character, the receiver doesn't require a highly accurate clock; it only has to be accurate and stable enough for the transmitter and receiver to stay synchronized to a fraction of a bit period of the time of one character, i.e., an accuracy of a few percent.

Note that conductor length plays a large part in serial clock errors. I stay under 19200 baud for regular ribbon cable more than 2' long. Makes me jealous of 4' 60Gb/s USB lines.

Electronic – Serial data transfer from Omega DP80 legacy hardware to computer

Here is how I would suggest going about debugging this problem:-

Check the connections.. if you've mixed up signal ground and chassis ground (shield) you could get funny behavior just like you describe. Pin 7 on the DB-25 is the one you want to connect to DE-9 pin 5. I'll assume you're using a commercial adapter/cable or have one wired like this.

Maybe check the transmit out line to ensure it's idling at -12 ( < -3 anyway).
Make sure the instrument is set to RS-232, and not current loop (Jumper 7,8 and 1,2).
Try the most common serial port setting (9600/8-N-1), 9600 baud, NO parity, 1 stop bit switch 1, 2, 3 OFF switch 4, 5 ON switch 6 ON switch 7, 8 ON

Make SURE you understand which is 'OFF' and which is 'ON'. Don't laugh, I've seen this happen more than once, and some types of DIP switches are kind of ambiguously marked. If in doubt, an ohmmeter across the switch.. in fact you could check all of the switches to ensure they're working.

I like Realterm as a terminal program. It's free, and easy to change the parameters. Try it on something known working first of course, and look for an online tutorial. Really worth it.
If still gibberish after that, get a scope screen cap or photo and post it.
If you have a continuing need for a logic analyzer, this one is very cheap and relatively useful. Not as good as my Tektronix boat anchor, but this one will fit in a coat pocket. But I'd get a scope first if you don't have one. Even the worst scope is better than nothing.

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Electronic – the maximum acceptable baud rate error for RS-232 serial communication

Electronic – Serial data transfer from Omega DP80 legacy hardware to computer

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