During initial testing of the MITLL2 CCD at Epping, it was realised that the limitation to readout speed of this device was the speed of the Serial Data Link between the CCD Controller and the Large External Memory. A modification of the Epping Serial Data Link was undertaken to increase this speed. This included increasing the link clock rate from 20 MHz to 25 MHz, and decreasing the number of bits transmitted from 32 to 16. These modifications permitted use of the MITLL2 "Normal" and "Fast" readout modes. (The link however was still not fast enough to cope with "Non-astro" ).
This link was sent to Site and placed in regular use for the MITLL2 from 16 October 1997. On 20 January 98, the three Serial Data links at Site were modified to allow them to be used with the MITLL2 CCD. During testing of these modifications, it was noted (by John Sullivan) that one link was generating errors (i.e. the link error signal was causing the Error status LED to come on). Further tests revealed that all links (including the Epping link) were generating errors, at a very low rate.
In order to assess how many errors the links were generating, a tests have been carried out of the speeded-up links between the AAT Vaccum Lab and the Control Room. A 1100 x 1090 = 1199000 pixel window was used.
Total errors (out of 1199000 pixels)
It should be noted that the type of "errors" occuring have not yet been quantified. The link error indicator can be generated as a result of a number of reasons, including an actual data parity error. Unforunately, to determine the type of error would require significant modifications to the circuitry. It has not been possible so far to determine if any of these errors are actual data errors. There have certainly been no reports of "suspect" data, and it is felt with confidence that there are no data errors occuring which result in values of either 65535 or 0.
A fix implemented on 14 February 1998 by Darren Stafford, resulted in this problem being cured. It no longer affects any observations (except for use of the NONASTRO mode of the MITLL2 CCD, which is not supported at present).
All observations taken with the MITLL2 CCD prior to 14 Feb 1998, and all observations made with all CCDs between 10 Jan 1998 and 14 Feb 1998 are potentially subject to these errors. However, the error rate is small (10 per 1 million pixels) and does not appear to produce any noticeable effect on data (certainly it was never noticed by observers prior to the John Sullivan's discovery of the problem). We do not beleive it to be a serious concern, but wish to make observers aware of what we have found.