Trying to Snoop an RS485 link - Port problems

Reply to Trying to Snoop an RS485 link - Port problems on Fri, 01 Oct 2021 02:23:11 GMT

Vladimir — Fri, 01 Oct 2021 02:23:11 GMT

Could you please share your wiring diagram? Or a photo of how you connect the tap?

For RS485 half-duplex (two-wire), you need to connect either the (RX+/RX-) pair or the (TX+/TX-) pair. The only difference it's going to make is how the data will be called in the IO Ninja log -- RX or TX. For RS485 full-duplex (four-wire) -- which is far less common -- you need to connect both (RX+/RX-) and (TX+/TX-). But you definitely don't need two taps to sniff a single serial link.

Reply to Trying to Snoop an RS485 link - Port problems on Thu, 30 Sep 2021 16:29:58 GMT

Walter Danti — Thu, 30 Sep 2021 16:29:58 GMT

Hi Vladimr, I was playing around a little more with your Serial Tap and note the following:
When I only connected the TX+ on your Tap the Modbus Analyzer recognizes both my Master request and my Slave's response. This is perfect and just what I wanted.

I am a little confused as to why the RX lines were not reading the data as I expected?

Reply to Trying to Snoop an RS485 link - Port problems on Thu, 30 Sep 2021 16:03:15 GMT

Walter Danti — Thu, 30 Sep 2021 16:03:15 GMT

Thank you Vladimir, that has worked. I do have a question related to Modbus data transmitted and received:
When I attached both the RX+ and RX- between my Master and Slave devices I only receive my Slave's response and the Modbus analyzer recognizes this. When I only keep the RX+ line connected I receive both thee Masteer and Slave data strings but the Modbus Analyzer does not recognize this. Am I doing something wrong or would I need a second IONinja Serial Tap to read both Masteer and Slave strings?

Reply to Trying to Snoop an RS485 link - Port problems on Tue, 28 Sep 2021 15:45:49 GMT

Vladimir — Tue, 28 Sep 2021 15:45:49 GMT

Oh, so you are using a Serial Tap for IO Ninja; not a generic dual-COM serial tap.

Then you need to use a dedicated "Serial Tap" plugin (not "Generic Serial Tap").

Reply to Trying to Snoop an RS485 link - Port problems on Mon, 27 Sep 2021 14:51:19 GMT

Walter Danti — Mon, 27 Sep 2021 14:51:19 GMT

I set up my serial Tap as you recommended in a previous post (RX+ and RX- on the TAP) and set up Buad rates etc to suit. Started the Snooping tool but got the Port messages above. What settings do I need to set to read both RX+ and RX- data?

Reply to Trying to Snoop an RS485 link - Port problems on Mon, 27 Sep 2021 06:27:50 GMT

Vladimir — Mon, 27 Sep 2021 06:27:50 GMT

Generic serial tap reads from two ports at the same time -- one port obtains RX, CTS, DSR; the second one obtains TX, DTR, RTS. Oftentimes, only TX and RX are required -- it simplifies the wiring. There are various over-the-counter dual-COM taps available; you can easily solder (or build on a breadboard) your own one, too.

The log error message says "ports A and B" while the toolbar refers to those ports as "DTE" and "DCE". This is confusing, I agree. We will change the port names in the log to "DTE" and "DCE" for consistency.