NI 9871 wiring to device with A and B terminals

[paperduck]

Hello,

I'm trying to install an additional device to a NI 9871 (4 port RS-485) module (in 2-wire configuration) to its 2nd port with no luck.  The module communicates fine with one other RS-485 device over modbus in my network that is connected to the first port of the module.  The device in question (a WattNode power meter - http://www.ccontrolsys.com/w/WattNode_Modbus) has 3 terminals: "A-" (which the manual states is RxD-,TxD- and "B+" (RxD+,TxD+ in manual) and a Common - the X terminal is there but isn't used. 

I tried twisting together wires from pin 4 (RxD+) and 8 (TxD+) and landed on terminal A- of the device, while tying together pin 5 (RxD-) and 9 (TxD-) and landed on terminal B+ of the device, with the Common terminal to pin 1 (the pin configuration of the standard DB9 that comes with the module, theother end being  RJ50).  This seemed like the correct orientation but I do not get a COM light on the device.  I tried reversing the wire pairs landed on A and B to no avail as well.

Some other notes:

1) The baud rate, parity, etc is all correct because I was able to get the device to an RS-485 to RS-232 convertor and communicate fine using the on board cRIO serial port.

2) It is a short run (about 10 feet) so did not use resistors at first.  But then tried a 125 ohm resistor (what I had on hand) that didn't solve the problem.

3) I configured the previous (working) device under the Project Folder and have double checked that I created the Modbus I/O server in a similar fashion but I'm also not sure if I need both these Modbus I/O servers under the same library or some other intricacy here.  

Thanks in advance for any responders!

[JefeL]

There are two things I would like you to verify. First, compare the setup with the configurations listed in the linked white paper:

What Pins Are Needed for 2- and 4- Wire Transmission with RS-485 Serial Communication?

Next, we want to verify the hardware is functional by establishing a single communication channel on Port A or on Port B. If we can get the devices to communicate on one port, and then switch the wires to the other port, it will tell us if there is a problem in the hardware itself.

Jeff L

Applications Engineer

National Instruments

[gwjtstengr]

Hi,

I'm beginning work on a project that uses a WattNode Modbus power meter connected via a USB-485 Serial Interface

and would be interested in hearing if you resolved your communications problem.  Like your situation, I have verified that

I can read registers from another Modbus device that I had available.

From your description, your RS-485 wiring and your serial settings seem correct.  I have some questions that I hope

might help:

  Does the specified Modbus slave address match the dip switch settings on the WattNode?

  Were you able to communicate with the WattNode using the first port that you already have working?

  Does your serial module have a "Transceiver Mode" setting?  (It should be set to "Auto" for two-wire RS-485).

Note, the "Com" LED on the WattNode will not light green unless the meter receives properly formatted packets.

It will light yellow, if it sees packets addressed to another Modbus device.

Regards,

gwjtstengr

[paperduck]

gwjtstengr,

Yes, I was able to figure it out in the end.  In summary, I was not able to get this working using the Server IO configuration, which was what I preferred and tried to figure out for a couple of days, but ultimately was able to get it working using the Modbus Library: https://forums.ni.com/t5/Reference-Design-Content/LabVIEW-Modbus-API/ta-p/3524019.

The library isn't supported very well and a little buggy, but if you know Labview you can work through it.  In order to use this library in scan mode
(as was my case), you'll need to install 9871 scan mode support https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P7gZSAS&l=en-US and also update the NI Serial version installed on the RIO.  Also, anyone that stumbles across this forum: be careful which software version combinations you install and make sure they are all compatible with eachother and your version of Labview.

Have you been able to get it working?

Regards,

pd

[gwjtstengr]

Hi PD:

Thank you for the update.  I'm happy to hear you resolved your Modbus issues.  I'm working with the MB Library

as well, so I really appreciate your confirmation that it works with the WattNode meter.

Previously, I have read data from holding registers in a different brand of energy meter using a modified version

of the MB Example Master VI.  So, I know I am able to perform simple Modbus queries via RS-485.

I plan to swap out an existing WattNode Pulse and install the Modbus unit within the next week.  I'm working on

program modifications to add basic configuration and read functionality for the new meter.

I will let you know how it goes.

[gwjtstengr]

Hi PD:

Installing the WattNode Modbus meter was pretty straightforward, since I already had wiring for a Pulse

meter in place.  And, I have now established serial communication with the meter.

Using the MB Serial Init and MB Master Query VI's, I created a simple VI to configure (write) the meter's

integer registers, and another to read them.  I was able to set the CtAmps to match the wired CT's, and

verify basic operation by reading the frequency and voltage registers.

The WattNode MB meter supports many of the register read and write functions, it is well-documented,

and fairly easy to install.  It provides both integer and floating point registers which makes it a bit easier

to get started (using the integer registers).

The MB Serial VI's appear to functioning properly.  So far, so good ...