RS485 with 7017

[JRussell]

We will be multi-dropping 3 7017 analog modules on a RS485 cable connected to a F1616BA. In one of the application notes for interfacing with the I-7000 modules there is a diagram showing a 9V power supply on the RS485 cable that is required when interfacing with a T100MD. Is this necessary when using the F1616BA, or is the F-series supply voltage through the port?
If it is needed, can that power supply come off the PLC, or does it need to be independent?
And finally, does the 7017 need to be powered separately through the (R)+Vs and (B)GND terminals, or does the unit receive power through the above mentioned RS485 cable?

[support]

The I-7017 requires its own power supply but it could share the PLC's power supply (24VDC).

the +9V shown on the diagram is through a pull-up resistor to the RS485 "+" signal. This is recommended if the RS485 line is long (tens or hundreds of feets) to ensure that the RS485 is set to a logic 1 when none of the RS485 transmitter is active.

If the I-7017s are near to the PLC the +9V pullup resistor may be omitted without affecting the communication quality.  

[TravisB]

I'm working with JR on this project so I'm going to continue a line of questioning in this thread.

We will be using the I-7017 at more than 10ft away from the PLC, so we will need the pull up resistor set-up. For the RS485 line power I see we need 9V, and I believe that can be 5-9V. I have my F1616 Analog ports connected to a AN20MA-2, so I've given up my 5V voltage source from the analog port. I think my next best option is to purchase a transformer to power the I-7017. I will have three of these I-7017 multi-dropped at different locations.
---What is the current requirement of the pull-up resistor set-up?

For this set-up I will also need resistors. The resistance I need is shown on the diagram, but my local electronics parts store also asks what wattage the resistor needs to be. I've tried to figure this out, but related to the above question, I don't know what currents I'll be dealing with.
---What wattage resistors do I need, or should I just buy them all at 1W to be safe?

I followed the TRi manual for connecting to an I-7017 using the U-485 USB to RS485 to connect to the PLC. The directions in the TRi manual can't be followed because the I-7017 website has since been updated. I downloaded DCON Utility, but the utility window looks nothing like the sample image in the TRi manual. I was able to connect, but I wasn't able to change the ID of the I-7017.
---Will you please update the TRi manual for the I-7017. At the moment I'm most interested in how works with the F1616, but foresee needing to use it with an FMD88 or other PLC from TRi.

Thanks!

[support]

At only 10 ft you may be able to get away without the pull-up/pull-down resistors. Run a test first and if you actually face problem then go setup the pull up/pull down resistors.

AN20MA-2 has a buffered analog reference output which can supply 30mA of current and you can use it to provide the pull up voltage via the resistor.

The resistors don't need to be big. You can use 1K ohm resistors and the maximum power it will dissipate is V^2/R = 5^2/1000 = 0.025W. So practically any resistor can be used (most common being 1/4W)

You can download the latest manual for I-70xx and the related software directly from ICP-DAS website at: http://www.icpdas-usa.com

In 3 to 6 months time we may be releasing our own analog expansion module that can connect up to 8 differential analog channels with 16-bit resolution via I2C bus. Look out for tech-tip and announcement emails send from TRi.

[TravisB]

I'm ready to take your advice and use the DAC from the AN20MA-2 to make the pull-up resistor to terminate the RS485. The termination will be 100+ ft away from the PLC. I have a few questions as to how I should proceed.

I have 8 strand CAT-5 cable that I am planning on useing to carry the RS485 signal. Can I use one of the wire pairs to also run power from the PLC to the Pull-up resistor? Or will this cause signal interference?

I believe that the pin on the AN20MA that I'm supposed to use is the Analog Ref. (AREF(out)),and that the complement is AGround.

I added the following line of code to the "1st Scan" rung.
SetDAC 7,2048 'Or some value larger than 2048

I then measured the output of wires connected to the ARef and AGnd with my voltmeter and didn't see any voltage or mAs, so I'm doing something wrong. I don't have any resistors connected, just trying to measure directly.

What am I missing here?

EDIT: I think what I'm missing is the power source to the AN20MA. In the AN20mA instructions it calls for power to the AN20mA to power the 20mA output. I have a transformer providing 24V 2A to the PLC. I think I need to connect V+ from the transformer to 24V on the AN20mA, and V- to 0V on the AN20mA.

[support]

Yes, you need to supply DC power to the AN20MA-2 in order to draw power from its Analog reference or 0-20mA outputs.

The CAT6 cable should work. Assuming DC current carry on the twisted pair is < 500mA it is unlikely to cause any interference to the RS485 twisted pair.

[TravisB]

Great news about the CAT cable, one less line to run.

The set-up as described above, using Aref, will only output 10.06V on a F1616-BA. To get 5V, I'm connected to DAC#4, and used the SetDAC 4,2048 command. ARef is not user selectable, it is either 5 or 10V, depending on the PLC.

Otherwise, I'm in business. Thanks!