readout and save times

[Chris Albertson <chrisja@jps.net>]

Tom Droege wrote:
> 

> 
> At the moment the worst time waster is the time it takes to read out the
> memory module and write a file.  This takes 100 seconds in QBasic on a 200
> MHz K6 to transfer 16 MBytes.  I assume this should take no more than 5
> seconds or so with machine code.

Five seconds may be a little optimistic but I think "effectively zero 
seconds" can be done.

Given a fairly modern IDE disk drive attached to the controller built into
most Pentium motherboards you can expect about 2MB/second sustained through
put to the disk.  This includes "real-world" things like the operating system
having to find free space on the disk and link all the sector into a file.
You will see much high advertised rates, up to 33MB/sec
but that only measures the transfer rate from the computer's RAM to the drive's
buffer cache the so-called interface rate.  So I expect it to take 8 second to
write out the file two FITS files. 

Readout of the RAM Buffer card may actually take longer.  The bottle neck will
be those 8-bit wide transfers over the ISA bus.  I don't expect more then 
1MB/sec.  So it is 16 seconds.  I would recommend installed _no other_ ISA
cards. in the real-time control PC.  Especially not an ISA Ethernet card
or disk controller or VGA card.  Any of these would compete with the RAM
card for bus bandwidth.

Many operators, I assume, will also like to see the images displayed on a
screen.  This takes time too.  As written, the image can go to the local
display screen or to a remote one.  This is likely another ten second
operation.

Then there is data reduction....

But the good news is that,  All these speeds are _unimportant_. 
We can readout the RAM card and write to disk
as background tasks.  So this processing to done in parallel with controlling
the shutters, drive motors and so on.  So for exposures spaced more than about
30 seconds apart saving the data will take effectively zero time.


-- 
   --Chris Albertson             home: chrisja@jps.net        
     Redondo Beach, California   work: chris@topdog.logicon.com