Overview A brief description of the Far Detector (and CalDet I) timing systems as I understand them.
This is a brief functional description of the current
implimentation of TimeMaster, the software running on the TPC.
A quick overview of how I see the timing system is shown below. (The
source image is bigger.. click to look at it.)
There are two bits of software: XNTPD, which acts as the network time server (using the Network Time Protocol NTP), and TimeMaster, which does the job of communicating with the timing hardware, communicating with Run Control, and doing all the logging and monitoring of the system.
Here's how the total system is supposed to work: The GPS provides an absoulte PPS (pulse per second) time to the TCU, and provides the date and time to XNTPD. The TCU sends to the computer (Timing PC or TPC) an echo of the PPS signal, so that XNTPD can lock the computer clock to this hardware signal. XNTPD then combines it's knowledge of the data (from GPS) and second (from the TCU) and serves these times out to the ROP processors, which then can properly timestamp detector data.
Meanwhile, the TCU provides two signals to the TRCs (the Timing Reciever Cards which sit in the VME crates). These signals are Buffer Swap and Execute. The buffer swap signal is used to syncronize the DAQ system so that data is written to one of two buffers while the other is being read out. The execute signals are used for triggering pedestal or charge injections in the front end electronics. The two signals are echoed by the TRC's onto the VME backplanes.
As of the time of this writing (July, 2001), the TCU (Timing Central
Unit) does not exist, so the figure below is the current kludge:
In this picture, there is no GPS, so a network time server is used to
find out the date, hour, minute. One of the two TRCs in this two-crate
system is a 'master' and is communcated to using a protocol similar to
the one envisioned for the TCU (although slightly less functional). The
master TRC then is electrically linked to the slave TRC, so they echo the
same signal somewhat syncronously. The software for this system
is nearly identical to the full one. This system is now up and working
at CalDet, and will be working at Soudan during the initial installation
phase. (The GPS, however, will be set up ASAP.)