VLBI Software Documentation
fmset: Set Formatter Clock
W. E. Himwich
NASA/Goddard Space Flight Center Version 9.3
Space Geodesy Program September 1, 1997
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.
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
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
(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
(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 &.