Manual Subreflector Chopping
			Bill Peters	   18 Oct 94

Overview:

	The subreflector is controlled via 5 CAMAC parallel I/O modules
in crate #3 which in turn are connected to 3 chassis of electronics mounted
below the crate.  The top module contains the logic for the chopping motions.
The middle module controls the stepper moters used to translate (focus) the
subreflector, and the lowest module is the power supply for the chopping motor.
All three chassis should be powered up for normal operation.  When the upper
module is turned on, it is typical for the "overcurrent" light on the left to
come on. Simply push the nearby button to clear this condition.  The chopping
electronics has the capability to nutate the subreflector to up to 16
pre-stored positions.  An external clock pulse is used to move the subreflector
from one pre-stored position to the next.  An incremental mode is also provided
where the subreflector moves by one anglar unit per clock pulse.  The chopping
range is approximately +-170 arcmin (+-8000 "units").  The maximum chopping 
frequency is approximately 30 Hz, although it cannot achieve this when chopping
at the maximum amplitude.  When using the incremental mode, the clock pulses 
should be much faster than this in order to result in a reasonable scanning
velocity.  (30 kHz?)

	The upper chassis has 3 BNC connectors on its front.  The leftmost is
labeled "instrument clock".  Use this for the above (5 volt) clock pulses.  (A
function generator is normally used for this.)  The middle connector labeled
"analog control" is normally unused.  An analog voltage here will overide the
above electronics and command the chopper directly.  The rightmost connector
labeled "position monitor" outputs an analog voltage corresponding to the
actual subreflector position.  (10v corresponds to about 8000 units.)  An
oscilloscope is often hooked to this connector to check that the subreflector
is moving as expected.  The chopping signal is also sent to the VFC chassis to
the right of Drumbeat on the receiver level.  When the subreflector is
chopping, one of the three input LED's (currently #2) should be blinking at the
chop frequency.  (Drumbeat is only expecting a two-position chop sequence.) 

Manual Chopping:
	
	The chopper can be controlled manually via the test program, TRYCHOP. 
Execute this from a terminal by typing: 
	    
	    	    RUN [SMT.TEST]TRYCHOP

	The program first asks if the current 16 memory positions are to be
zeroed.  Answer Y unless the memory has already been loaded.  The recognized
commands are:

	P n1 n2 n3 ...      Define sequence of positions n1 n2 n3 ... 

where n1 n2 n3 ... are up to 16 positions to load into memory.  Minimum is
-8192, maximum 8191 "units", where 8000 units is approximately 170 arcsec.
For example, "P 100" defines a "null" chop sequence, namely, stay at position
100.  "P -2000 2000" defines a two position chop sequence at -2000 and 2000
units.  (I think the subreflector will not even move to the first P command
position until the B command [below] is executed.)  Executing the P command
will also stop any scan or sequence in progress.

	B		    Begin a chop sequence on next clock pulse
	E		    End the sequence or incremental scan
	I+ or I-	    Begin a scan incrementing in + (-) direction
	R		    Read current chopper memory position (1-16).