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I have a system that is a Windows 10 laptop that needs to connect to some systems using a 56k modem. The modem itself is a standard US Robotics 5686 (E, I believe) external modem. It works on other computers.

The Windows 10 laptop obviously didn't have a serial port, so I connected a Sabrent CB-9P6F USB to serial cable to the USB hub connected on that laptop. Nothing works. I can install the modem manually and choose COM4 or COM5, for instance, but I can't even query the modem. It's like it's not even connected.

Back when I tried to get it working on a Windows 98 computer with no networking support, I had fewer issues in trying to get it working than I am here.

If it was just that Windows 10 was so awful that the support for this was gone, that would be one thing, but the specifications for both the modem AND the adapter say Windows 10 is supported, so what gives? How can this be made to work, if at all?

In other words:

  • drivers are not an issue
  • modem support for Windows 10 is not an issue
  • adapter support for Windows 10 is not an issue

The adapter does show up in Device Manager as a COM port; I'm just not able to make use of it. I can add a modem manually (Windows can't automatically detect any) on that port, but it doesn't work at all.

After installing suggested fix:

Still doesn't work, unfortunately. These are the major changes I noticed: enter image description here

enter image description here

The modem is on COM4, and COM4 displays the yellow warning icon and complains that the driver was not digitally signed, etc. However, I'm not sure why this would prevent the modem from using it.

InterLinked
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  • The USB to serial adaptor should appear as a COM port, you then talk **through** that COM port using standard serial communications to the modem, you don't need a new device for the modem. – Mokubai Jan 23 '20 at 19:44
  • As Mokubai said, you need to communicate using the COM port that appears when the Sabrent adapter is plugged in. Do you get a new port when it is plugged in? – Natsu Kage Jan 23 '20 at 19:47
  • What does Device Manager show? Do you get “This Device Cannot Start (Code 10)” for the device ? – harrymc Jan 23 '20 at 20:06
  • How are you connecting to this modem (which typically has a DB25 connector)? Modems require proper connection of DTR/DSR/RTS/CTS/DCD signals (aka modem handshake lines) with the DTE port (e.g. the USB-serial adapter). The proper connections rarely happen by accident. You may need to use a breakout box or a voltmeter/DMM to verify signals. – sawdust Jan 23 '20 at 20:39
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    A COM port is added. However, I can't connect to the modem using that COM port. I'm using a USB to DB9 adapter and then the cable that comes with the modem appears to be serial (DB9) and parallel (DB25) to plug into the modem. – InterLinked Jan 23 '20 at 22:34
  • @InterLinked *"parallel (DB25)"* -- See https://superuser.com/questions/547374/is-db-25-port-serial-or-parallel/547660#547660. *"I'm using a USB to DB9 adapter"* -- An issue I've had with some USB-serial adapters is that pin 5 of the DB9 (which should be the ground connection) is not actually connected to anything. Use a multimeter to check for continuity (or not) between pin 5 and the shroud of the USB connector. A possible workaround is to ensure that the PC and modem share the same power strip. IOW do not run the laptop off the battery; use the AC power adapter. – sawdust Jan 24 '20 at 00:01
  • @sawdust The laptop is always plugged in and yes, they share the same power strip. Daisy chained together, but they're actually on the same strip. – InterLinked Jan 24 '20 at 02:46

1 Answers1

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The Sabrent CB-9P6F device is made by Prolific, which is the source of the problem.

The article Prolific USB To Serial Driver Code 10 Fix has an explanation for the problem:

What has happened is that there have been counterfeit “Prolific” chips coming from China. The counterfeit chips use the same Vendor ID (VID_067B) and Product ID (PID_2303) as the authentic Prolific chips. So, Prolific made a new chip and dropped all support for the old one thereby rendering the counterfeit chips unusable. Unfortunately, it renders hundreds of legacy devices using the earlier chip inoperative. Getting a working driver installed by the average user is almost impossible.

If one of these Prolific drivers gets installed to your Windows 64-bit computer then your legacy device will no longer work and will issue the generic “Code 10” error.

Or, you may get no error at all, but your device will not work.

The simplest solution would be to get a non-Prolific USB to serial cable.

If you prefer instead to get this one cable working on Windows 10, you should install an older driver.

You may find such a driver on this webpage. The version you want is "PL2303", so download the version 32/64-bit that suits your Windows version.

harrymc
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  • If no joy, install a serial port add in card. – Moab Jan 23 '20 at 21:41
  • What's the difference between PL2303_64bit_Installer and PL2303_64bit_Installer_1? The latter is a slightly smaller file. Unfortunately, there's no read me or description – InterLinked Jan 24 '20 at 02:50
  • I have no idea. The article recommended `PL2303_64bit_ Installer.exe` so try it first. I suppose that if it doesn't work, you could try the other. – harrymc Jan 24 '20 at 07:07
  • @harrymc OK, I tried what you suggested, see edits to post. Progress, maybe?? Not sure. Still not working, but at least I get errors now! – InterLinked Jan 24 '20 at 21:06
  • In [this post](https://www.supportrix.com/kb/how-to-install-unsigned-drivers-in-windows-10/) try the method in the section "Installing Unsigned Drivers by Enabling Test Mode" in order to reinstall the driver, but do not undo Test Mode for the moment while testing if the device now works. – harrymc Jan 24 '20 at 21:19