Meinberg VME31 - Manual (Page 8)

8
Selecting the I/O Access Mode
An additional jumper J1 must be configured to select the access mode: if the jumper is
installed, the board can be accessed using Short Supervisor I/O (2Dh), otherwise the
board is accessed using Short Non-Priviledged I/O (29h).
Installation
After the jumpers have been configured, the board can be installed in the VME bus
rack. When the system is turned on, the clock starts displaying either the on-board date
or time, depending on the position of the date/time selection switch. The LEDs let the
user check the clock's status and proper receiver operation.
It is important to position the antenna in an optimal way. The antenna should be
installed at least 30 centimeters away from the clock board and from solid steel. It
should be aligned at a right angle to the direction of the transmitter (Frankfurt).
If the antenna is connected, the brightness of the green LED labeled Feld reflects
the strength of the RF signal. A good way to align the antenna is to turn it slowly until
this LED is mostly dimmed due to minimum signal, finally, the antenna is turned by a
90° angle to obtain maximum signal.
If the antenna is properly installed, the green LED labeled Mod. should be blinking
exactly once per second corresponding to the time marks from DCF77. If this LED
flashes, there is some electrical noise around which prevents the receiver from
decoding the time marks and synchronizing.
The red LED labeled Freil. is on if the clock is running on xtal. This LED can only
change when the minute changes (seconds increment from 59 to 0). After power-up, it
takes at least two up to three minutes until this LED is turned off.
Interface Description
Information on date, time, and status is made available to the VME bus via dual port
RAM. The dual port RAM is mapped into the VME bus' A16 address range correspon-
ding to the jumper settings described above. The contents of the dual port RAM is
updated once every 10ms.
Care must be taken not to read an inconsistent block of data due to an update cycle
occurring in the middle of the read loop. For example, the 100th-of-seconds value
could be read twice: as the first byte, and, again, as the last byte. If both of the values
are identical, no update has taken place while the read loop was executed. Another