title:		How to Replace a PSC Module
date:		6/17/2003
by:		zms

1.	Overview

A Power Supply Controller (PSC) device is a single width CAMAC module, designed and
built at SLAC.  It has a high precision 16 bit ADC.  The 16 bit module is labeled
PSC-II, to be distinguished from its earlier 14 bit ADC version, labeled PSC.  This
module can fail in a variety of ways, the most common seems to be loss of DAC -
analog voltage linearity, which is most likely the cause when the magnet system
has difficulty reaching a set point.  Number of other failures are possible.
Each PSC module has a S/N engraved at the bottom of its face plate.  Each also has
a SLAC Id sticker.  If replacing the PSC module does not fix the problem, the
problem may be caused by another module in the CAMAC crate or the crate itself.

2.	Replacing a PSC Module

The following steps should be adhered to:

 a.	Identify the location of the defective PSC in the magnets CAMAC crate in
	building 108 in the research yard.
	If you know which magnet circuit has the faulty PSC, you can mouse click
	on the magnet name in the Magnets display to obtain the magnet circuit
	properties.  The properties display shows details of each component in
	the magnet circuit.  It shows the CAMAC slot where the PSC is located
	on the first line of text describing the PSC module.  This line shows
	toward its end (CAMAC n=xx), where xx is the CAMAC slot number.
 b.	Quit the mags program.  You do it by pulling down the File menu and
	clicking on the Quit entry.
 c.	Power down the magnets CAMAC crate.  There is no A/C power switch.  You
	power it off by unplugging the A/C cable from the back of the crate.
 d.	Obtain a working replacement module.  You could use one of the PSC modules
	in the magnets CAMAC crate which are not used for this experiment.
 e.	Disconnect both cables from the module that is being replaced, remove the
	faulty module, and insert the replacement module in the slot from which
	the faulty module was removed.  Reconnect both cables which were previously
	connected to the faulty module.
 f.	Write down on a piece of paper the S/N of the replacement module and the
	module being replaced, which you will record in the logbook entry.
 g.	Make sure that the module was correctly inserted and the cables properly
	connected.  Power the crate on by plugging the A/C cable in the rear of
	the crate.  You should see the bypass LED light on the crate controller.
	Now you are finished in Bldg 108.
 h.	Back in the counting house, you run the magnets program and observe whether
	the new PSC module is being read properly.
 i.	Assuming that all went well so far, one more task still remains.  You need
	to verify that the DAC calibration coefficients that were used with the
	faulty module work well with the new module.  You may need to modify the
	calibration coefficients.  The procedure for obtaining and modifying the
	calibration coefficients is outlined below.

3.	DAC Calibration Coefficients

The PSC module DAC device accepts a number from 0 to 65768.  The DAC converts the
number to an analog signal, a voltage in the range from 0 to 10 V.  This voltage
tells the Power Supply controlled by the module to supply appropriate current to
the magnet.  The current flows through a transductor, which supplies a signal to
the PSC module.  This signal is read out via the PSC's ADC device and is shown in
the Magnets display as primary or secondary current.  The signal from the "cage"
transductor gives us the primary current.  The purpose for obtaining the DAC
calibration coefficients is to make sure that the software calculates correctly
the DAC set point corresponding to a desired current set point.

The relationship between the current set point i and DAC set point dac can be written
as:
	dac = (i - do) * dc
where do is the DAC offset or origin (dorg) and dc is slope (dcal).  To obtain the
DAC calibration coefficients follow these steps:

 a. In MagsControl dialog window turn off checking (the Check toggle indicator
    must be off).
 b. Set the DAC to a set point, wait until current reaches its corresponding value,
    record both the DAC set point and the resulting primary current.
 c. Repeat point b. until sufficient number of points have been collected to
    cover the working range of the magnet circuit.
 d. Estimate if the points obtained fall on a straight line and calculate do and
    dc using the above formula.  Compare with the old coefficients.
 e. Modify the calibration coefficients in the mags program, on how to, see below.
 f. Verify that the new coefficients work.  This is done by running the circuit
    to some current set point.  The magnet circuit should reach the current set
    point without wandering around.

4.	Modify the DAC Calibration Coefficients

There are two ways you can get new coefficients into the magnets (mags) program.
One is to modify the mags.dat setup file, which defines all spectrometers and 
all magnet circuits for each spectrometer.  Initial values of various parameters
come from this setup file, among which are the DAC calibration coefficients for
each PSC module.  The other is to modify these coefficients via the "Modify
Magnet xx" dialog.  This dialog gets mapped when the Modify push button is pressed,
located in the Magnet column in the MagsControl dialog.  Note, that using the
Modify dialog to enter the new values of the coefficients does not change the
contents of the mags.dat setup file.  You can write the current configuration data
to a new disk file via the File/"Save As..." menu entry.

5.	Contents of the CAMAC Crate in Bldg 108

At the time of this writing, the CAMAC crate in Bldg 108 contains:

	Slot	Magnet	PS	PSC	SALC Id
	N			S/N
	3	Q4	PB201	95	
	4	D2	PB202	23
	7			47	18000118
	9	D3	PB204	163
	11	Q3	PQ201	26
	13	D1	PQ202	62	
	15	Q1	PQ203	84	
	16	Q2	PQ204	27	
	18			51	18000123
	20			215	18997071
	21			4	18000045
	22			?	18000469