Use cFP RS-485 port in 2-wire mode

[Wayne.C]

Good day all.

Getting ready to go where NI suggests that I shouldn't go.    Has anyone used the 485 port on a cFP controller for 2-wire communications?  I know I can externally tie RX+ to TX+ and RX- to TX- together and as long as I deal with the "echo" of my transmitted data, it should work in theory.  My question is has anyone out there found any other gotcha's?  Not too worried about noise because this 485 network is less that 20ft long. 

thanks for your input

[Elizabeth_S]

Hello,

National Instruments does not support using compact FieldPoint with the RS-485 port in 2-Wire mode.  If you still want to do so, you can respectively short the RXD+ and TXD+ lines and the RX- and TX- lines on the RJ-45 connector.  If you run into problems, National Instruments recommends using a RS-232-485 gateway or purchasing a separate Ethernet, USB, PCI, PXI, or PCMCIA RS-485 adapter which natively supports 2-Wire mode.

Can I Use the cFP-2020's RS-485 Port in 2-Wire Mode?

http://digital.ni.com/public.nsf/allkb/E110B51DC18C8D2EC1256C9F007552AE?OpenDocument

Elizabeth S.

Applications Engineer

National Instruments

[Wayne.C]

Elizabeth,

I'm well aware that NI does not support 2-wire mode on a cFP and I'm sure that you are just as aware that users of NI don't always stay within the bounds of what is supported.  Is there a specific reason for not supporting 2-wire mode?  Is NI worried about possible hardware damage to the 485 port?  Problems with noise?  Can you find out if the cFP 485 port implements the termination resistors internally?

A note about my application.  Target cFP is acquiring data from its I/O modules and other instruments in system using RS232 and 485.  Laptop host used as portable HMI for operator.  Only connection between laptop host and target system is through wireless network.    USB, PCI, PXI or PCMCIA adapters are not an option.  If this doesn't workout I'll build a 232 to 485 adapter.  I have a design I've been using for several years now.  I just hate to clutter up my system with another widget.

[Elizabeth_S]

Hello,

The main reason that National Instruments does not support using compact FieldPoint with the RS-485 port in 2-wire mode is because of the related transmission errors. 

If you are using the cFP-2020, then there are no internal bias resistors on the RS-485 port.   

Does the RS-485 Port on a cFP-2020 Have Internal Bias Resistors?

http://digital.ni.com/public.nsf/allkb/01AE447531680CD986256FDA007A4100?OpenDocument

Please post back if you are interested in further information about hardware damage and noise.

Regards,

Elizabeth S.

Applications Engineer

National Instruments

[Wayne.C]

Elizabeth,

Thanks for the reply.  Safe to assume the 2120 port is the same as 2020?  The fact that the resistors aren't internal is perfect for my purpose.  I'm really not too worried about noise with such a short cable run and slow data rate (9600 baud).  We also happen to produce a line variable frequency drives that include RS485 ports and have a lot of experience with electrical noise in severe environments.

The only real concern would be possible long term damage to port.  I feel better knowing port is not terminated internally.  The only other thing I can think of that might come into play is what kind of hardware protection scheme NI implemented in the port.

[Elizabeth_S]

Hello,

We do not know if there would be any long term negative effects of the RS-485 port in 2-wire mode.  This type of application has not been validated and is therefore not supported.  

When you ask about the hardware protection, I assume that you are referring to isolation for the port.  The RS-485 port is isolated for up to 100 Vrms during normal operation.  Please see the following knowledgebase article.

Is the RS-485 Port on the cFP-2020 Isolated?

http://digital.ni.com/public.nsf/allkb/C6E889706FD6C07A86256C600060256C?OpenDocument

Elizabeth S.

Applications Engineer

National Instruments

[Wayne.C · Accepted solution]

I ran some tests and finally found the gotcha and the real reason 2-wire mode is not supported on cFP 485 port.

I have a cFP-2120 running with the 485 port in 2-wire mode and connected to two instruments.  Program was previously tested using laptop and USB-485 adapter to make sure it was rock solid.  Program reads each meter, and keeps track of any errors that occur.  Funny thing, one meter read perfect %100 of time (0 failures out of 5427 tries).  The other meter failed about %3 of the time ( 172 failures out of 5427 tries).  In each case it was a framing error. 

Since one meter always reads correctly, I ruled out noise and went looking for bus contention.  Scope revealed that the problem is how fast the cFP port tri-states its output buffers after a TX.  Turns out that one meter is responding a little quicker than the other and that its reply is getting clobbered by the cFP port.  Oooops! 

In this case I'm out of luck and on to PlanB.

[Mads]

Why do NI call it an RS-485 port when it does not support 2-wire? Is not RS-485 always 2-wire? I would expect them to call it an RS-422 port when it operates with 4.

If there was an Idea Exchange for hardware I would ask NI to fully support RS485. Keeping the number of wires at a minimum is often more important in the industry than having full-duplex communication...and PACs are now more and more often used in applications where space and power consumption are limiting factors so adding a converter to get proper RS485 is not always an option. I would also ask that they do what a lot of others already do - namely make all the serial ports switchable between RS232 and 485....and of course - make more units that have more than one in-built serial port, for us the cFP-2220 is the one and only PAC because of this.

We currently use a cFP-2220 embedded into an intelligent sensor and have to have at least one 2-wire RS485 connection. We can do that today even though the switching to receive mode is so slow because we can control the response time of the nodes as well. If that was not the case we would have to look for an alternative to the PAC (lower power consumption, wakeup functionality ++ are other issues that would be great to get NI to look at on the PACs).

[Wayne.C]

Mads,

Like you, I find the lack of 2-wire support in the cFP 485 ports a pain.  The funny thing is that this is the only NI product line where I've seen this particular problem.  I'm now starting to use cRIO's instead.  At least with them I can add 4 rs232 or 4 rs485 ports at a time.  

I like your product suggestion about the ports that can switch between 232 and 485 operation.  Please go ahead and submit that to NI.

[JasonSmith]

I know this is a very old topic, but I just ran across it. I wanted to point out that the cFP-2220 does fully support 2-wire RS-485 communication on the RS-485 port. The cFP-20xx and cFP-21xx were unconfigurable as mentioned in this post.

To configure the wire mode used with the cFP-2220, it is necessary to modify the niserial.dbs file which is in the /ni-rt/system folder on the target. There are instructions in the NI-Serial help file which explain how to do this.

-Jason S.