USB RS232 Serial com port adapters

Discussion in 'Frequently Asked Questions (Public)' started by Colin Haig, May 18, 2021.

  1. Colin Haig

    Colin Haig Staff Member

    Oct 27, 2014
    Are you having problems with Windows COM: ports, serial devices, data communications equipment, or certain pieces of astronomical equipment?

    This article touches on a few common issues and solutions related to RS-232-C serial ports, COM ports, USB to RS232 adapters, and devices that behave as Virtual Communication Ports.

    Background Information:
    The Electronics Industry Association defined a data communication standard called RS-232-C that allowed for reliable data transfer over fairly long cable lengths. Older computers equipped with serial communication ports, also known as COMM ports or COM: ports had an actual RS232 communication port, built onto the motherboard or on an expansion card. They had a DB-25 or DE-9 connector (sometimes called D-shell). RS232 allowed long cable lengths because it used +/-12V for signalling. More detail is available in this Wikipedia article here:

    Common Serial Port Equipment:
    Items in the typical observatory that may use RS-232-C serial devices or Virtual Com Ports include:
    • Modem for data communication (less common today than 20 years ago)
    • Mouse to move the cursor/pointer
    • MaxDome II dome controller
    • Older SBIG cameras like the AllSky 340 or ST-6
    • NMEA Global Positioning System (GPS) devices
    • Telescope mounts
    • Dome or roof controller
    • Weather sensors
    • Focusers
    Over time, RS232 ports were no longer included in PCs, and a new technology called Universal Serial Bus (USB) came standard. Note that the S in USB has very little to do with Serial Port - it's just that the data flows in a stream of bits serially, rather than in parallel.

    Universal Serial Bus allows much faster data transfer, however is restricted to short cable lengths. Typical devices include:
    • Keyboard and mouse
    • Thumb drive (USB memory stick)
    • USB to RS232 serial adapter* to convert from USB to RS232
    • Boltwood Cloud Sensor II
    • SBIG CCD and CMOS cameras
    USB to Serial Adapters
    To connect your old RS232-based devices like a modem, serial mouse, or telescope mount, you needed an adapter that converts the USB standard to an RS232 serial communication port, known as a USB to Serial adapter.

    Devices that adapt from USB to a RS232 DE-9 connector use semiconductor "chips" made by many manufacturers, and convert the USB signals and protocols to PC communication port standards and an RS232 interface.

    USB Native Devices
    Some products are natively USB, and don`t need an RS232 adapter, but do use the same underlying serial protocol. Examples include the:
    • Boltwood Cloud Sensor II
    Each manufacturer may provide a device driver and software to control their equipment.
    It is a good idea to visit their web site to download the latest drivers and software, and then install them.
    If you are still having problems, then read on.

    Chip Makers and Adapter Makers
    There are several manufacturers of semiconductor chips used to make USB to RS232 serial adapters and USB native devices. The manufacturers often provide a basic device driver that allows Windows, Mac, and Linux computers to talk to their components.

    Here are several device makers and and a link to their device drivers:
    We recommend that you check with your hardware manufacturer first as they may have a customized driver version. The ones listed were current at the time of this writing.

    Quality and Support Issues
    If you have a problem with serial communication, a better quality adapter may solve your issues. Note that some devices are not designed for use outdoors and may fail or become unreliable at outdoor temperatures. A number of users report good success with these adapters:
    • FTDI Chip based adapters such as the Star Tech ICUSB2321F. You must disable Serial Enumeration, Disable modem control on startup, and turn off USB selective suspend eg do not allow Windows to turn off the device to save power.
    • Tripplite Keyspan USA19HS
    Windows Update Issues
    Windows may automatically update drivers unexpectedly. This means it might accidentally install an incorrect driver version, or when it updates a device driver, default settings get turned back on, and you need to fix this. I recommend keeping a copy of the known-good working driver in your downloads directory in case you need to uninstall and reinstall.

    Windows upgrades can also cause this problem. Windows 10 major updates such as Creators Edition or 20H2 can re-install device drivers, and this has led to problems with devices not working. Check each device in the device manager.

    Device Manager (devmgr.msc) disappeared with a Windows 10 update (late 2017, Creators Edition). It is no longer available from the usual Settings Gear, Settings, Devices. You can still run it with File Run devmgr.msc or WindowsKey+R devmgr.msc. You might want to make a shortcut on your desktop.

    Solving the Jumping Mouse Cursor Problem
    Windows will automatically test all serial COM ports for an attached Serial Mouse. This will play havoc with some astronomy devices. If your mouse cursor starts jumping around and you can`t get control, unplug your USB/RS232 adapter.
    It uses a facility called "Serial Enumeration" to test if there is a mouse attached to the COM port, and it will send characters and expect a response. If the right combination of modem control signals and characters come back, it will automatically add a Serial Mouse. You can find this in the Device Manager and disable the serial mouse. The problem is getting control of your machine when this happens. Again, unplug the device. In remote observatories, this can be very hard to do, but keyboard navigation may work.

    To prevent this from happening in future, make sure you disable any Serial Mouse listed in Device Manager.
    Then make sure you turn off Serial Enumeration in your USB/RS232 adapter Advanced settings, if they are available.

    If it is not available, it may be found as an Upper Filter in the Registry associated with the device. It is tricky to find and fix that, and the use of the Windows Registry Editor is beyond the scope of this article.

    Preventing Disconnects
    Windows power management can turn off USB devices, including serial ports and virtual com ports. Here's another post on our forum to help solve those issues:
    Last edited: May 19, 2021

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