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I am attempting to connect to a weight-measuring scale using the following communication line: Laptop -> USB to DB-9 Serial adapter -> Scale. The problem I'm experiencing is that my laptop recognizes the adapter as the device on the COM port, thus when I check the VID and PID of the device, it gives me the IDs for the cable and not the scale. Is there some way to find the information for the device on the other end?

I am coding in Python using the PySerial library, but any general answer would be helpful. The adapter I'm using is a USB to UART PL2303 from Prolific Technology Inc. My OS is Windows 10.

I have tried connecting scales from multiple different brands using a USB-to-Serial adapter, and I get the same result. I haven't been able to establish a connection with any of them, with all queries to the scales not being responded to. Two such scales are the Sartorius Entris and the Mettler Toledo Excellence Plus.

EDIT: Responses are saying that serial devices do not have a VID or PID. In that case, how can I tell if a serial device is a certain device from a specific manufacturer? I'm trying to do this so I can recognize when a scale is plugged in and not some other device, using a Python program to communicate with the scale.

Noah Mazza
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  • Information about a connected device over an RS-232 serial link would only be obtainable if that device chooses to provide it using whatever high-level protocol or command/response dialogue it supports. "*my laptop recognizes the adapter as the device on the COM port*" -- You have it backwards. USB is a bus; USB connects the serial port to the system bus. The VID & PID are USB attributes for the microcontroller that performs that interface, i.e. the RS-232 adapter. The OS considers the USB adapter with its DB-9 connector the boundary that it directly manages/controls. – sawdust Feb 18 '23 at 01:53
  • It would be helpful if you could share some more information about the scale (maybe a manual or at least model number). Does the scale also have a USB connection as well? Why do you need the PID and VID, because as @sawdust said above, those are USB attributes. Is it also USB-TMC device? – Misplaced Southerner Feb 18 '23 at 07:49
  • "*how can I tell if a serial device is a certain device*" -- Already answered. Did you read my comment? Did you study any operating and/or programming/interface manuals for the devices that you have? Your goal of "*recognize when a scale is plugged in*" is simply not practical with a simple RS-232 connection. That's why other PC peripherals such as printers and scanners have evolved to prefer the USB interface for the convenience of users. It's industrial/commercial peripherals that have stuck with the inexpensive & simplistic RS-232 interface. – sawdust Feb 18 '23 at 22:19

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Serial devices dont have VID/PID pair.

In "historic" times PCs have two ports - parallel and serial (also known as RS-232). Serial devices are PnP devices, so dont need driver and because this no need VID/PID pair.

What scales are you used? If industrial, is very possible that are based on RS-485 standard.

Jozo121
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  • A low-quality answer with half-baked information. PCs can have more than just "*two ports*"; what about the video port or game port? You mischaracterize "*serial devices*", "*PnP*", and the "*need [for] driver*". – sawdust Feb 18 '23 at 01:35
  • Other ports are not relevant to the Op question. – Jozo121 Feb 18 '23 at 05:12
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    I added the scales used to the comment. They are the Sartorius Entris and Mettler Toledo Excellence Plus. – Noah Mazza Feb 18 '23 at 18:27
  • @Jozo121 "*Other ports are not relevant to the Op question*" -- True, so why then do *you* bother mentioning parallel port in *your* "answer"? – sawdust Feb 18 '23 at 21:50
  • @NoahMazza, thanks for giving some more information. Would you be able to get a more specific model number off of one of the devices? I tried googling "Sartorius Entris", and it seemed like there were many different versions. Furthermore, one I took a deeper look at only had a USB-C connection, so you'll definitely need to narrow it down. – Misplaced Southerner Feb 20 '23 at 07:06