Hi, USB is no use for any time critical application. We put considerable effort into using it for an industrial automation type I/O application a few years ago and it was a waste of time and effort. The protocols are non-deterministic and the data is packetised so timing is inconsistent. There are problems with even "simple" USB to RS232 applications (I have experience of problems on motion control, Development systems and instrumentation (Gamma Ray Spectrometer). Some adaptors work better than others (The FTDI chipsets work better than most), but even if you find one that works, small changes in the computer configuration can cause havoc. It only takes someone to plug a USB device with poor drivers into the system to mess everything up. When you come to fault-find, the offending device has often gone, but the damage has been done.
Robert G8RPI. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Hal Murray Sent: 11 August 2006 21:05 To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] PPS over USB (Was SiLabs) > Is the USB bus really a good way to get 1 PPS into a machine? I would > think it would have quite a bit of jitter in it. I'm not a USB wizard, but I work with people who are. There is no interrupt mechanism on a USB cable. The host/master polls the devices/slaves. Typical numbers are 10 ms or 1 ms. At setup time (I forget what USB calls it), the device can tell the host how often it wants to be polled. (I assume the host can ignore that or round up to what it can handle.) For a homebrew project, it might be possible to turn things inside out. Have the host flap a signal, say from the printer port, and the GPS device records the time. Then the device reports that time via USB. One of the popular GPS over serial port devices works that way. (I forget which one.) -- The suespammers.org mail server is located in California. So are all my other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited commercial e-mail to my suespammers.org address or any of my other addresses. These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Any opinions expressed in this email are those of the individual and not necessarily Genetix Ltd (Genetix) or any company associated with it. This email and any files transmitted with it are confidential and solely for the use of the intended recipient. If you are not the intended recipient or the person responsible for delivering to the intended recipient, be advised that you have received this email in error and that any use is strictly prohibited. If you have received this email in error please notify Genetix by telephone on +44 (0)1425 624600. The unauthorised use, disclosure, copying or alteration of this message is strictly forbidden. This mail and any attachments have been scanned for viruses prior to leaving the Genetix network. Genetix will not be liable for direct, special, indirect or consequential damages as a result of any virus being passed on, or arising from alteration of the contents of this message by a third party. _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
