Countrate data in "Restricted Imaging" format.

Niels Lund

DSRI JEM-X Technical Note #10. Revision 2

990423

1) Background

In the restricted imaging format the timing information for individual events is lost, and

thus the timing information is limited to the image sampling rate which will typically be

between 8 and 24 s. The need for better timing data have been stressed on several occasions,

and it has been decided to set aside a fraction of the telemetry to provide a single count rate

channel with a time resolution better than 1 second. This note suggests an implementation scheme with a separate Count Rate Packet which may also be applicable to the pure spectrum format.

2) Imaging collection time in Restricted Imaging format.

Restricted Imaging (RI-) format will normally only be used when the average rate of good

X-ray events is about 100 cps or higher. For lower rates the Full Imaging format will be the

natural choice. The maximum rate of good events which can be transmitted in the current

RI-format is about 300 cps, corresponding to images containing 2400 events, transmitted in

8-packet sequences. Therefore we can conclude that the integration time for an image in the

RI-format will normally lie between 24 and 8 seconds, dependent on the instantaneous rate of

accepted events.

3) Timeout of data collection in Restricted Imaging format

It is important to limit the time the instrument will wait for the 2400-event buffer to fill up during data collection. If the detector high voltage disappears or the strong source is occulted by the Moon the system should not hang forever, waiting for the last 100 events to arrive. We have decided to implement a 32 s timeout value for the data collection for a single Restricted Imaging image.

4) Count rate data in the RI-format

In the current EID-B the RI-format is defined as having 382 bytes of imaging data and 24 bytes of countrate data in each data packet - in other words we have accepted to use about 5% of the telemetry capacity for the count rate information. We expect to use 8 such packets per image.

5) Expected number of counts per channel

We anticipate that the maximum rate at which the CPU in the DFEE can process good events

is around 2000 cps. At this rate of events we will already have a very appreciable deadtime.

Therefore, if we settle for a time resolution of 125 ms we will expect a maximum of about

250 counts per time channel. We will encode the differences between neighboring count values, these differences will normally fit within one byte.

6) Proposal for a simplified packet format

It has been realized that the proposed format containing both image and countrate data within

the same packet is cumbersome both to generate and to decode. Moreover the description of

possible changes to the greyfilter value is not complete in the existing format. We therefore

propose to separate the two types of data in each their own packet type (with different APIDS).

JEM-X Countrate packet format:

APID: JEM-X unit 1: 1100 1110 000 JEM-X unit2: 1101 1110 000

Word 0,1,2:	OBT of start time of first channel in packet (1/8192 s precision)

Word 3:	bit 0-7: Greyfilter valid for first channel (ch. 0)

bit 8-15: = 0 (channel number for which grey filter takes effect)

Word 4-10:	Room for new greyfilter values (if relevant).

bit 0-7: New Greyfilter (only valid if bit 8-15 nonzero)

bit 8-15: channel number where new greyfilter takes effect

Word 11:	Counts in first time channel

Word 12-212:	Count value differences (see note) for approximately 400 time channels

Each channel is 125 ms wide.

Packet data corresponds to approximately 25 seconds of input data.

Word 213:	CRC

Under nominal conditions we will have one science-tm-packet per second. Every 25 s we will transmit a countrate packet when operating in RI-format. Thus only 4 % of the data is used for this purpose. In fact, the new proposal allows to transmit more image data than the current baseline format.

Count rate packets will be generated continually as long as the RI-format is active. If the

formatting reverts to Full Imaging or changes to another format which includes fine timing

information, then the current count rate packet will be padded with 0xFFFF and transmitted to the telemetry, after which the generation of count rate packets will be stopped.

The start of a new time channel in a count rate packet will always correspond to a value of the JEM-X onboard clock fullfilling the condition:

Clock Reading modulo 1024 = 0 (this occurs every 125 ms)

This means that the time channels in any two Count Rate packets are phase locked - even if

the packets are separated by a period where no Count Rate packets were generated.

It is required that the start and stop times for every image transmitted in the Restricted Imaging format are subject to the above condition of the Clock Reading. This ensures that the number of events in any image exactly matches the number of events in the corresponding sequence of counts in the count rate packet stream.

It should be straightforward to implement the same type of countrate packet when the

instrument is using the pure Spectral Format.