When in FILING transmission mode, an instrument transmits heartbeat messages over its data port. These short messages take the place of data blocks, and ensure that programs such as Scream know that an instrument is present. Heartbeat messages are output at regular intervals and always have the same format. Older digitisers may not include all the lines described here.
Heartbeat message structure
An example of a heartbeat message is:
Flash memory usage
The first three lines describe the internal Flash memory.
First, the overall usage is described: 64MB Flash File buffer:
Here, a 64 Mb Flash memory store contains 65,520 data blocks, of which none have already been downloaded. This leaves 16 blocks of free space, since each block is exactly 1Kb long, and (65,520 + 16) x 1Kb = 65,536 Kb = 64 Mb.
These two lines show the date and time of the oldest and newest data in the buffer, together with the System ID (PLPGG) and the Stream IDs (SBHYX2 and SBHYN4) of the corresponding data stream.
The next line gives the date and time of the last trigger, together with its System ID and Stream ID as above. Also given is the event sequence number (15, here) as used by the EVENTSLIST command, and the position in the buffer where data from the event begins:
If a trigger has occurred, but no data was output (e.g. because the digitiser was in DIRECT transmission mode), the digitiser will report:
The next line describes the current status of the GPS system. When the digitiser is first powered up, the message will be something like:
The message GPS control OFF denotes that the clock is not yet being controlled by the GPS. Owing to noise purposely injected into the GPS timing stream, as well as issues of signal travel time, the GPS time cannot be directly used to synchronise the clock. Instead, the digitiser maintains a rolling average of time measurements, and uses this to apply corrections to the clock so that it remains accurate over a long period.
The Auto 3-D message denotes that the signal is strong enough to obtain a full 3-dimensional GPS fix. If the signal is weak, or there are too few satellites visible to the receiver, you may see Auto 2-D here. If the signal is too weak to obtain a fix,
will be displayed.
After a short time, the message will change to:
Eventually, the GPS control system will be able to synchronise the internal clock, and the message will change to:
The system is now reporting the current offset of the internal clock from GPS time (whether slow or fast), and the current frequency error. It will now continuously adjust the internal clock for as long as the GPS is powered up.
If you have chosen to save energy by enabling GPS power cycling, the DM24 will switch off the GPS once a satisfactory fix is obtained, and begin free-running on the internal clock. Whilst this is happening, the line will report
These three lines repeat information from the last re-boot:
- Line 1: The System ID , the Stream ID of the status stream, and the current Sensor Type
- Line 2: The number of times the digitiser has been power cycled, and the number of times it has been reset without power cycling (e.g. by a RE-BOOT after re-configuring the module: a soft reset)
- Line 3: The date and time of the last power cycle and of the last soft reset.
The next line displays the current state of the digitiser’s power supply, and its internal temperature:
The final line provides the instantaneous mass positions reported by the sensor at the time given:
2006 1 18 14:56:15 Mass positions -486 -300 -424
Status blocks in continuous mode
In continuous mode, each status block contains a number of single-line messages:
The messages appear in the order they are generated by the different software modules inside the digitiser.
GPS status messages
The status of the attached GPS receiver is reported every minute:
In this line:
- o/s gives the current offset of the internal clock from GPS time, in arbitrary internal units;
- drift is the difference between the current o/s and the last, which gives an indication of the level of drift;
- pwm, or Pulse Width Modulation, is the feedback control parameter;
- Auto 3-D denotes that enough GPS satellites are currently visible to obtain a full 3D fix. If, instead, No Fix is displayed, the GPS received has not (yet) been able to obtain a satisfactory fix. If Missing PPS is displayed in this field, the digitiser has received fewer than 60 pulses on the 1pps (pulse per second) line in the last minute. You will see at least one Missing PPS message in the first minute of operation.
Every 20 minutes, at 10, 30 and 50 minutes past the hour, the GPS synchronisation status is displayed. This line has the same format as the GPS line in the heartbeat message:
Finally, every hour, the DM24 displays general timing and position information, including details of the visibility of GPS satellites:
SV# stands, officially, for Space Vehicle number. Each GPS satellite has it’s own unique identification number, or SV# , and the list shows which are currently usable (satellite numbers 4, 7, 13, 20, 23, 24 and 25 in this example). A minimum of four is required to obtain a full 3-dimensional GPS fix.
When a trigger occurs, a message is immediately saved to the current status block. You may not see this message straight away, however, since the status block must be full before the digitiser will send it:
The trigger type is mentioned, as well as the sequence number (for later retrieval, if necessary, using the EVENTSLIST command). When the trigger condition ends, the system reports:
These are measured every 10 minutes (at 5, 15, 25, 35, 45 and 55 minutes past the hour), and are given in the same form as for heartbeat messages: