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Old September 11th, 2019, 11:42 AM   #1
walkir
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Tech How to connect USB device to PLC via RS485

I have several scales with USB output.
How can I connect it to my PLC that has RS485 input?
There are hundreds of USB to RS485 converters but all of them are designed to connect RS485 device with PC. I need something that is working in the opposite way.
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Old September 11th, 2019, 01:05 PM   #2
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I doubt your going to find anything. This isn't just a matter of converting RS485 to USB, it's a matter of bridging the communications protocol that the scale is communicating to the protocol that the PLC is communicating (along with bridging RS485 to USB).
Having said that, if it was possible you would need to provide us with information on the scales (make, model, communications protocol) as well as the PLC's information.
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Old September 11th, 2019, 01:44 PM   #3
walkir
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Unhappy

This is what I was afraid of. I have Panasonic FP0H PLC with RS485 card, but I don't know what kind of USB protocol is the scale using. I hoped for some universal solution that I could use with different scales.
Basically I need to measure the weight of different ingredients dispensed at differents points and hoped to use market available scales but I might end up with load cell and some custom solution.
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Old September 11th, 2019, 02:55 PM   #4
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Your best bet is going to be using a scale that is designed to integrate with a PLC and possible a specific brand. For example, if you are using a Rockwell PLC, go to their Encompass Partner section and look up "weighing / Instruments". You'll find companies that make scales that (in most cases) will interface with Rockwell PLC's. The other option would be to find a scale the communicates Modbus (RTU and/or TCP) which is a protocol that most PLC's can work with and if not, it's easy to find Gateway's that will bridge Modbus to several different protocols. There are a lot of ways to do what you are looking to do but starting with a scale meant for the Industrial Automation world will save you a lot of time and headaches.
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Old September 11th, 2019, 03:11 PM   #5
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Model and Brand of the scales would be helpful.
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Old September 11th, 2019, 04:19 PM   #6
Ken Roach
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How exactly does the USB port function on the scale ? Does it emulate a keyboard (an HID device), or does it implement a virtual serial or Ethernet adapter ?

Most PLCs don't act as a USB Host. Some HMIs do, so you can connect a keyboard or mouse to them.

A Red Lion DA30D protocol converter has a USB Host port that can support keyboard/mouse devices for data entry. That might be one way to get your scales interfaced with something that has a protocol in common with your Panasonic PLC.
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Old September 11th, 2019, 06:38 PM   #7
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Quote:
Originally Posted by Maxkling View Post
Model and Brand of the scales would be helpful.
It is Brecknell Model PS25
Usually, it is connected to a PC or RasberryPI with proper drivers.

Here is driver description and configuration:
https://www.brecknellscales.com/down...B%20Driver.pdf
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Old September 11th, 2019, 07:04 PM   #8
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The USB connection seems to actually be serial, so you may be in luck.
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Old September 11th, 2019, 07:27 PM   #9
Ken Roach
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The USB implementation of a virtual serial port is notoriously driver-specific even in Windows.

Unless he wants to take apart the scales and find the output of the FTDI or Prolific chip and substitute an RS485 chip, I don't see an easy solution.

If there are drivers for Linux for these scales, then a Raspberry Pi might be a reasonable piece of middleware.
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Old September 11th, 2019, 08:05 PM   #10
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Quote:
Originally Posted by Ken Roach View Post
The USB implementation of a virtual serial port is notoriously driver-specific even in Windows.

Unless he wants to take apart the scales and find the output of the FTDI or Prolific chip and substitute an RS485 chip, I don't see an easy solution.

If there are drivers for Linux for these scales, then a Raspberry Pi might be a reasonable piece of middleware.
Yea, I just find it weird they are taking some form of protocol (probably something very generic and usable) and internally converting to a USB bus and then require a virtual serial driver to render the data. The scales are cheap and I wouldn’t hesitate to poke around, especially near the USB port. Now obviously Frankensteining a scale probably shouldn’t be an enterprise or production solution.

I guess in my head it would make sense to have the scale output a serial signal and offer the end user a converter to usb offering options. I recently worked with a GSM relay that was CAN BUS and came with a converter for a virtual serial driver. That gave you two options, integrate with CAN or use a terminal.
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Old September 12th, 2019, 09:05 AM   #11
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That scale isn't meant for industrial Automation applications, it's meant for calculating postage. The reason it's uses a virtual serial port is so it can support older versions of software used to calculate postage as well as new versions. That's an industry all to itself with it's own communications protocols so it would stand to reason that there isn't a quick, easy way to adapt it to Industrial Automation applications.
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Old September 12th, 2019, 09:35 AM   #12
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Quote:
Originally Posted by Maxkling View Post
I guess in my head it would make sense to have the scale output a serial signal
You mean like the poorly named PS-USB?? Which has Serial (RS-232) AND USB?

https://www.brecknellscales.com/prod...es/ps-usb.html

Edit: The manual for the PS-USB also lists the comm protocol to be used by the scale, which should be the same between versions, although that's not really the problem you're facing.
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Old September 13th, 2019, 11:16 AM   #13
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Thank you for all help and suggestions.
After a few days of analyzing this protocol, I have given up on this one.

Unfortunately, I need 10 scales communicating with PLC and the cheapest with RS-232/485 I have found was for $300...

I have a very good electrical engineer who can design an amplifier so we might end up buying cheap scales and get read of its electronic, salvage load cell, connect to our customized amplifier and connect it to an analog (V) input of PLC.

In case anyone knows affordable scales that have a reasonable way of communicating with PLC please give me a hint. That would save us some time.
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Old September 13th, 2019, 11:35 AM   #14
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Anything industrial related won't be cheap unless you take chance on fleabay. $300 actually sound quite reasonable.

Maybe take a step back and tell us what the function is. What kind of action will the PLC take based on the scale reading?

If you are going with a hack-scale route, there are instructions on the net.

https://hackaday.io/project/1242-giv...ale-serial-out
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Old September 13th, 2019, 12:11 PM   #15
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Get a small (maybe cheap) PC (like lenovo ideacenter similar to https://www.newegg.com/lenovo-ideace...82E16883108711)
Run AdvancedHMI. Connect to your PLC.
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