ASCA GIS-3 cpu trouble

[Fumiaki NAGASE <nagase@astro.isas.ac.jp>]

Dear ASCA team members and ASCA users:

Recently we are noticed by the KSC operation scientits and a few
observers that the pulse hight spectra of GIS-3 are somehow strange.
Phenomenologically, pulse hieghts of ADC out accumulated in every
(8n+5)channels of the 10 bits registers (the lower 3 bits were not
operating correctly but fixed), resulting a 128 channels spectrum
instead 1024 channels. 

During the KSC passes of last Friday, we have checked and found
this was not due to the hardware but due to the on board CPU
that processing GIS-3 pulse signals. It was found that a two-bit error
in the cpu memories occurred in a strange manner that do not
completely breakdown the software but make the above error.

GIS team examined the observed data and found that this two-bit error
in the GIS-3 cpu had happen on February 10, 1994. Unfortunately,
we have continued observations with this unconforatble mode during
the period, only 128 channel pulse height resolution is avalable
for GIS-3 for the interval from February 10 to April 8. Yet I 
think that the 128channel spectrum (energy width per channel is about
100 eV) gives reasoanble energy resolution for GIS spectrum.

A report on the details of this malfunction of GIS-3 will be issued
later by the GIS team. It seems that two bit errors in GIS computer
happen more frequently than the theoretical estimate. Hereafter
we will include memory check of the GIS CPU stationally and more
frequently in the daily operations.

Regards,
Fumiaki