VLBI Software Documentation

Field System

fmset: Set Formatter Clock

W. E. Himwich

NVI, Inc./GSFC

User Guide

NASA/Goddard Space Flight Center Version 9.3

Space Geodesy Program September 1, 1997



Table of Contents












1.0 Introduction




The fmset program is an interactive utility to allow setting the time for Mark IV and VLBA formatters and S2 recorders. It is necessary to use software for this operation because these formatters have no physical controls to allow the time to be set. The program only allows the time to be set to the one second level. The time within one second is determined by the formatter or recorder from a 1 PPS signal.





2.0 Operation



fmset is a separate program, not a SNAP command. It may be run from any shell prompt. It can also started by typing Control-Shift-T or selecting Fmset from the operator's menu at the window display if the default window manager set-up is in use. The Field System must be active before fmset is started.

If fmset is used with a VLBA formatter, the formatter must have had its address set using the form=addr SNAP command before starting fmset. The program determines which type of formatter it is working with based on the type of rack that was specified in the equip.ctl control file when the Field System was last started.

The fmset program should not be left running continuously. It should only be run for long enough to set the formatter time or occasionally, to check it. Besides the danger of accidentally changing the formatter time, the real-time display of the formatter time, especially for VLBA formatters, uses a great deal of the communication bus bandwidth. If the program is left running it may interfere with other Field System functions.

The fmset program presents an interactive display with three different times: (1) the formatter time, (2) the Field System time, and (3) the computer time. It is important to understand the distinctions between these times. The formatter time is obviously enough the current time in the formatter. The computer time is the time reported by the PC-FS computer operating system. The Field System time is the corrected computer time being used by the Field System. The fmset program is used only to set the formatter time. The other times are displayed for information only and may or may not be related to the UTC time. The displayed times update once a second, roughly as the seconds unit digit of the formatter changes.

There are five basic commands, independent of the hardware being used, available from the display, each is accessed by typing a single character:

(1) to increment the formatter time by one second type + (plus),

(2) to decrement the formatter time by one second, type - (minus),

(3) to enter an arbitrary time to set the formatter to, type = (equal),

(4) to set the formatter time to the current Field System time, type . (period),

(5) to terminate the program press the Esc key.

There are two additional commands that depend on the type of hardware being used. For a system with a Mark IV formatter, the formatter can be resynched by typing s. For a system with S2 recorder, fmset can be toggled between setting the S2 recorder clock or the Mark IV/VLBA formatter by typing t (when the S2 recorder is in use it's time is the more fundamental since it is the time recorded on the tape).

Whenever the time is set for the S2 recorder, by any of +, -, = or . commands, the station delay will be set to zero if the delay measurement is non-zero.

Please be aware that in using these command there may be small timing variations that affect the results. For example, if in response to an increment command the new time arrives at the formatter just after the change to the next second, no change will occur. Similarly, when the formatter is set to particular time using =, it may end up a second off.

The command = can be used to set the formatter to an approximate time. In response to =, the display stops updating and the user is prompted for the year, month, day, and time to use. The day of year can be used by specifying the month as 0. Just pressing Return in response to any prompt will cause the current value to be used for that field. After the time has been prompted for a final request for confirmation is made. Either a y to accept the value or n to abort the operation can be entered. The input is not accepted until the user presses Return. The command . can also be used to get the formatter approximately correct if the Field System time is approximately correct.

The increment + and decrement - by one second commands can be used to adjust the formatter in small steps. These commands are intended to provide fine control over the formatter time. To move the formatter time by more than one second these commands can be given by repeatedly pressing the appropriate key. It is recommended that the key not be held down, distinct key presses give more control. After incrementing or decrementing the time it is useful to check the display to verify the results. If several increments or decrements are sent quickly it may take a few seconds for the display to begin updating again. Once the display is updating normally, you can check whether or not the formatter time has been adjusted as expected. Because of the timing problems alluded to previously, the adjustment may not exactly reflect the commands given. It may be necessary to "trim" the setting a little more.





3.0 Procedure for Setting Formatter Time



Use the following procedure to set the formatter (Mark IV, VLBA or S2) time:

(1) Synch the formatter (or S2 recorder) to the 1 PPS signal. The procedure for the VLBA formatter is described in the VLBA hardware manual, it would normally include rebooting the formatter.

(2) Start the Field System, if it isn't running already.

(3) If you are using a VLBA rack, send the formatter its address with the SNAP command form=addr.

(4) Start fmset, either from a new window or by typing Control-Shift-T or selecting fmset from the operator's menu.

(5) Use = or . commands to get the formatter time approximately correct.

(6) Use + and - commands to "walk" the time in to the correct value. You should compare the displayed formatter to a local station clock to verify that it is in synch. It is particularly important to make sure that the unit digits of the seconds displays agree between the formatter and the station clock. If there is a hardware time display on the formatter it should be used for the comparison to the local station clock. This is preferable to using the fmset displayed time, but the results should be the same.















(7) Terminate fmset with an Esc command.

(8) Reset the Field System time by issuing the SNAP command: sy=run setcl offset &.