Bias and ground in RS-485 connection from cFP2020 to FP1001

[bob_miller]

I am implementing an RS-485 connection over 2000 feet between
a cFP-2020 and a FP-1001.  In an earlier post I mentioned that
lightning-induced voltage surges may be an issue (each building has its own
ground reference)

Two issues arise as I study how to wire this connection correctly.

1) Neither the cFP-2020 nor the FP-1001 biases the RS-485 bus.
     NI recommends a resistor network of a 680ohm pullup and pulldown
     to +5 and GND for each pair.   **Where is the +5 supposed to come
     from??**

2) There is a "GND" pin on both the cFP-2020 and the FP-1001 RS-485
     ports.  How should this be wired (e.g to shield at one end, both ends,
     each wired to local GND,  left unconnected, etc...)

Bob Miller

[bob_miller]

OK, here's what I think is what's needed, after doing a search elsewhere (gee,
the silence here is deafening )...

I will bias both the tx and rx pair at the cFP2020 end with 620 ohms to
ground for one wire and the combination of 3.3K to 24V and 860ohms to
ground for the other. This, in addition to the 100 ohm termination resistor
across each pair at both ends. should provide the required 200mV idle
bias.

Additionally, I will wire the GND pin between the two fieldpoints' RS485 ports
with an unused pair on the cat5e and tie the pin to earth at the cFP2020
end through 100 ohms.  My MTL RS-485 surge protectors at each end  provide
for protection of this GND wire in addition to the four data wires.

Its kind of a shame the cFP2020 doesn't provide this bias functionality like the
FP-1000 does...I hate little "dingleberry" breadboards in my nice clean
control box!!!

Bob

(confirmation that this is indeed proper wiring for my RS-485 bus would
still be greatly appreciated!!!)

[Hal_L]

Hi Bob,

I was wondering if you could provide me with some additional information on your setup.

1.  Are you using 2 wire or 4 wire communication?

2. Would it be possible for you to post a schematic?

3. Are you using duplex or full duplex communication?

The following could be helpful in the meantime:

- Serial Quick Reference Guide

Regards,

Hal L.

[bob_miller]

I'll try to provide as much information textually as I can since I have no idea how to
post a graphic image on this forum in a form everyone can read. Hopefully with a
pencil and paper you can draw the plan from the text that follows:

-- I am using 4-wire full duplex RS-485, since that is the configuration NI seems to
      want me to use. I have not yet written the LabVIEW code for the cFP2020
      to communicate over this bus. I understand it is not "quite" automagical (e.g. MAX
      doesn't just find the FP1001and its I/O devices and vanilla FP read/write vi's work).

-- Both TX and RX pairs will be terminated at both ends with 100/120ohms (some
     RS-485 docs  recommend 100, NI suggests 120). Temination resistors are wired
     between tx+ and tx-, rx+ and rx-.

-- The cFP2020 is the master in the network; the FP1001 is the only slave. At a later
      time a second  FP1001 slave may be added.

-- Because the FP1001's isolation specs are more robust than the cFP2020's, I will
      do the biasing at the '2020 end. NI suggests wiring tx+ and rx+ to 5V through 680
      ohms, tx- and rx- to GND through 680 ohms. At least I assume both pair need to
      be biased.  But in my case (and probably most everyone else's case) there is no
      +5 supply. Therefore I will wire bit tx+ and rx+ both to GND through ~860ohms and
      24V through 3.3k. The Thevenin equivalent of this resistor pair is about 680 ohms to  5V.

-- While RS-485 is differential, the common mode needs to be referenced, particularly
      for isolated interfaces.Thus the GND pins on both ends will be wired together through
      an unused pair on the cat5e. I find no documentation about this in the NI site, but
      other RS-485 documents  such as the B&B Electronics "Troubleshooting Guide for
      RS-485" at  www.bb-elec.com strongly  recommend it.  And (unless I am
      misunderstanding something),  this signal ground needs to be referenced to earth at
      at least one point, and a 100ohm series resistance to earth is recommended "to limit
      ground currents".

-- Finally, at each end of the bus, all four RS-485 signal wires plus the ground wire will be
      connected to the interfaces and termination networks through MTL surge protectors
      designed to shunt spikes over +/- 15V to local earth ground.  While the isolation of the
      interface on the FP-1001 is probably good enough for most surges, it seems prudent
      to protect it further and to protect the bus from differential surges in this lightning-prone
     area -- particularly since the two fieldpoints are 2000 feet apart in two different buildings
     each with their own electrical service and earth ground reference.

Whew. Hope that clarifies everything a bit.

Bob

     

[bob_miller]

By the way, there is at least one error in the "Serial Troubleshooting Wizard" document
referenced earlier: the bias resistors should force the voltage between the differential pair
to be "at least" 200mV , not "less than 200mV".  200mV is the minimum spec'ed RS-485 logic
threshold; anything less may result in indeterminate interpretation at the receiver.

Also, my apologies for continually saying 680 ohms when I should be saying 620. I think I'm
dyslexic or something.....not that the precise values are necessarily that critical.

Bob

[Hal_L]

Hi Bob,

Thank you for the quick and thorough response.

If you want to terminate both ends of the cables, all you have to do is connect a 100 Ohm or 120 Ohm resistor between all signals with a + and - line.  For example: with an RS-485 DB-9 connector:  you would want to insert a resistor between:

1. CTS+ and CTS-

2. RTS+ and RTS-

3. RXD+ and RXD-

4. TXD+ and TXD-

Termination between different lines from different lines to 5V and Ground is taken care of inside the module itself.

National Instruments also makes a terminating connector.  So instead of soldering 4 resistors to each end of the cable, you could purchase 2 of these and connect everything together.  Here is a link with more information on these: DB-9 Terminating Connector for RS-485 NetworksDB-9 Terminating Connector for RS-485 Networks.

Regards,

Hal L.

[bob_miller]

---

@HAL L wrote:

>>Termination between different lines from different lines to 5V and Ground is taken care of inside the module itself.

The NI knowledge base article entitled "Does the RS-485 Port on a cFP2020 Have Internal Bias Resistors?"
answers this question "No." Based on that data I have assumed I must bias. (If I don't, I'd be delighted...)

I assume the cFP2020 doesn't bias because it might end up on the same bus with other cFP2020s, in which case
the bus will be over-biased. This isn't an issue with the FP1000, which has bias but is guaranteed to appear only
once on the bus.

Hence my complaint about no +5V to bias with. Most installations will only have +24V lying around.

Bob

[Hal_L]

Hi Bob,

You are absolutely right: the cFP-2020 does not have internal bias resistors. You will have to add your own resistors externally to create the biasing.  You will need to supply the +5V externally. 

Regards,
Hal L.