At 12:28 PM 3/8/2002 +0000, Carlos Parada wrote: >Hi all, > >I would like to use the RTLinux with a GPS antenna (Trimble) for a very >precise synchronisation. However, I'm very confused about what can be made >in RTLinux, what kind of protocols can be used and which pins should be used >to transport the PPS signal.
What do you really want to do? The one pulse per second output of the GPS is only good to from +/- 250 ns RMS for one brand or +/- 40 ns for another brand of GPS. To synchronize, or time-stamp an event with a UT (universal time) from a tracable clock, you will need to build or buy a PCI bus card with and FPGA and inputs and outputs, and write firmware that your computer can talk to (real time linux probably not needed) We have built this kind of hardware for our experiment at the south pole. If you have a 50 MHz local clock on your FPGA board you can time stamp to about 20 ns. If your FPGA has a PLL frequency multiplier in it, you can time stamp to a clock tick of the fast clock in the fpga. You can do even better if you differentiate the edge of the one pulse per second signal, and digitize the data stream. Then you use a simple program to find where the edge of the pulse departs from base line. We achieve synchronization to less than 2 ns with a remote receiver on a cable over 2 kilometers long (rise time of 1.2 us). It's possible to do better with a faster rise time and less system noise than we have. It wouldn't make much sense to time stamp to a few ns with the dismal time precision of the GPS. So we actually lock to the one pps output of a Rubidium oscillator which is slaved to the GPS. We get a few orders of magnitude improvement that way. (see http://www.srsys.com/html/prs10.html, for instance) >I'm studying different scenarios of configuration and I would like to know >if they are possible or not. >The better solution for me (less expensive) is the possibility of >synchronises the PC through a RS323 cable, receiving a TSIP and PPS signals. The time string from the GPS receiver is read by the serial port of the computer. The timestamp for the event comes from reading some registers in the FPGA board that does the time stamping. If you are an educational institution, the Altera company will help you with the FPGA and the software tools to program it. >However, I don't have reached any information about the support of the >RTLinux of this capability, or any way to make it support. >The solution of receiving the NTP and PPS signal via RS232, is also a If you build your own FPGA card, which you may have to do if you want to put some ADCs on it, and signal conditioning, then you will find it much easier to design and talk to an ISA card than PCI. Don't build a PCI card unless you already know all the traps. good luck j >possibility (more expensive). Anybody knows if either the RTLinux supports >this, or need any software more. > >I'll thank very much some information and tips. -- [rtl] --- To unsubscribe: echo "unsubscribe rtl" | mail [EMAIL PROTECTED] OR echo "unsubscribe rtl <Your_email>" | mail [EMAIL PROTECTED] -- For more information on Real-Time Linux see: http://www.rtlinux.org/
