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USB Serial RS232 Communications on Windows - Preventing disconnects and errors
Have you experienced communication errors, equipment disconnects, poor reliability, or erratic mouse pointer operation?
A common cause is related to how Windows and Mac handle RS232 serial devices, USB devices, and USB to RS232 serial adapters.
Background Information
For more information on RS232, there is an excellent WikiPedia article here:
https://en.wikipedia.org/wiki/RS-232
Older computers 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.
RS232 was commonly used to connect devices such as a:
- Modem for data communication
- Mouse to move the cursor/pointer
- Telescope mount
Over time, RS232 ports were no longer included in PCs, and a new technology called Universal Serial Bus (USB) came standard. USB is common to Windows PCs and Apple Mac computers, as well as Raspberry Pi and many Linux devices.
Universal Serial Bus or USB lets you plug in any number of devices such as a:
- keyboard
- mouse
- thumb drive (USB memory stick)
- USB to RS232 serial adapter* to convert from USB to RS232
- Boltwood Cloud Sensor II
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:
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:
Quality and Support Issues
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 in the advanced driver settings, and turn off USB selective suspend eg do not allow Windows to turn off the device to save power. More on this later.
- Tripplite Keyspan USA19HS
Silicon Labs devices have exhibited problems for some users - You may want to update to the latest driver, and keep earlier driver versions if you run into problems.
Prolific Technology suffered as there were counterfeit devices being sold. They changed their drivers so that older and counterfeit units no longer work. Many users have experienced poor reliability and compatibility problems with older Prolific PL2303 based devices, and these are often sold very inexpensively and under many brand names. Star Tech, Staples, Best Buy and many others sell inexpensive devices that have PL2303 family chips, and some users experience problems. Make sure you get the right device.
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 reinstall many 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 recent 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.
For example:
Attachment 9357
You may also need to turn off "Modem Control at Startup" - I haven't done so in the screen shot.
Some device drivers don't show this option. It can be found as an Upper Filter in the Registry associated with the device. It is tricky to find and fix that, and beyond the scope of this article.
Communication Errors and Windows USB Power Management
Some users encounter errors with failed or unreliable communication to their serial or USB devices.
A common cause is Windows power management "helping" by shutting off power to USB devices, hubs, and root hubs.
Although this is good for the environment and energy savings, it is not good for reliability of your equipment.
To solve this, you need to disable Windows Power Management - USB Selective Suspend.
You may also wish to change your Windows Power Management plan to "High Performance"
Here are some common places to check, all reachable through the Device Manager.
You must check this for EVERY COMx: port, Serial Converter, USB Hub, USB Root Hub, and USB Host Controller.
Check that "Allow the computer to turn off this device to save power" is OFF / Disabled in the USB Serial Converter Properties:
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In the USB COM port itself:
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In the USB hub it is attached to:
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In all the USB root hubs (the topmost hubs, closest to the computer, in case you have multiple hubs or active extensions):
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In the USB Host Controllers (built in):
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There will be other variations on the device drivers on your particular Windows PC, be sure to check them all.
Communication Errors and Cabling and Hubs
Bad cables are the #2 cause of USB communication errors.
Make sure you are using the shortest possible cables that don't snag or drag, and comply with USB specifications.
USB 1.1 and 2.0 cables have 2 pairs of wires. The data pair is typically 28AWG, and the power pair is 24AWG. Lower numbers are better. eg. 24+20AWG is better than 28+24AWG, but is a stiffer, more expensive cable.
These cables are shielded with foil, and sometimes it is worth sacrificing a suspect cable to the wire cutters to see what is actually inside. There have been examples of cables that do not meet the specifications. Certified cables will have a tag (as shown on the left edge of the www.USB.org page) stating the cable is USB, USB Hi-Speed, USB SuperSpeed, or USB SuperSpeed+. Printing on the cable itself may state what the cable is, however, the certification is a more trustworthy.
Cable lengths vary by USB speed.
USB 1.1 maximum length is 3m (9 feet 10 inches)
USB 2.0 maximum length is 5m (16 feet 5 inches)
USB 3.0 maximum length is 3m (9 feet 10 inches)
You can add a limited number of hubs or active extension cables (1 port hubs) to extend these.
Powered hubs are always recommended. Passive hubs just don't get the job done.
Some laptops do not meet all the USB specifications for power levels, and so you may need to add a powered hub.
There are many good quality powered hubs avaiable.
Some users report success with these Star Tech industrial hubs: https://www.startech.com/ca/Cards-Adapters/USB-2/Hub/Mountable-Rugged-Industrial-7-Port-USB-Hub~ST7200USBM
Many hubs do not operate well in lower temperatures, due to ceramic resonators/oscillators shifting in frequency. These oscillators control the hub timing. Check the operating temperature range of the device.
The Star Tech example above is rated to 0C (32F), although it has been known to operate at -20C (4F) by some users.
Problems Mixing and Matching USB Devices and Speeds
It is a bad idea to put high-performance devices that monopolize the USB bus on the same bus segment as lower speed devices. Specifically, DSLRs, Webcams, and CCD Cameras should have their own, independent, direct connection to the PC. These devices will fully utilitize USB 2.0 speeds (480 Mbps).
Older cameras that do not have frame buffers (such as the STL-11000M) require full use of the USB 1.1 bus. Dropped lines, bad images, or garbage data will be the result of bus over-utilization.
It is a bad idea to mix devices of different speeds, as the USB bus will tend to slow down to the performance of the lowest-speed device. So, put all your USB 1.1 devices on their own bus. Things like keyboards, usb/rs232 adapters, focusers, and telescopes tend to run at these lower speeds.
There are utilities like USBview that will list all USB devices, what ports and bus they are on, and what speed they are running at, and their power consumption. This can be helpful when you are trying to find that independent port for the camera or put the low speed devices together. It will also help you see the power consumption of the devices, and so you can adjust what is plugged in where. The SBIG ST-i is a nice autoguider that has a mechanical shutter, and will draw a fair bit of power, almost a full 500mA, which is the total that some USB ports provide. This camera should either go on its own port or on a powered hub.
Some devices say they are USB 2.0 compatible, but run at low speed (like USB 1.1). Generally you can mix and match these, however USBview will show you what is really going on.
Notes
Your individual setup and operating system version will have an impact. This article will hopefully help with a few of the pitfalls, and feel free to contact the author.
