signal enable/disable in drvSIS3820 driver

[Hovanes Egiyan <hovanes.egiyan@gmail.com>]

Hi,

when we were trying to use SIS3820 struck scalers with mca record on a Linux CPU
using modified EPICS mca module, we run into a problem when we could not enable all inputs
(signals) of the Struck scaler module in EPICS, which is probably the most frequent 
mode for this scaler readout. We tracked it down to the line# 480 in drvSIS3820.cpp asyn driver
for SIS3820:

/* Disable channel in MCS mode. We enable the first maxSignals_ inputs */

registers_->copy_disable_reg = 0xFFFFFFFF << maxSignals_;

The result of the left-shift of a 32-bit variable by 32 bits in C apparently is undefined, and it
happens to be 0 on our Motorolla VME controllers running VxWorks, and 0xFFFFFFFF on
our Intel-based Linux controllers. Therefore, when we were enabling all input signals, the
driver for Linux was actually disabling all of the signals. We patched it using an if-statement and it
seems to work for us now for our customized version of the driver, but it would probably be useful to do
something similar in the official driver. 

      disableMask = ( allEnabled << maxSignals_ );
      if( maxSignals_ >= SIS38XX_MAX_SIGNALS ) disableMask = 0;
      vmeWrite32((volatile unsigned int*)&registers_->copy_disable_reg, disableMask );

The same issue probably will arise at line# 521 (SCALER mode):

/* Disable channel in scaler mode. */

registers_->count_disable_reg = 0xFFFFFFFF << maxSignals_;

Best,
    Hovanes Egiyan