User manual

Page 5-1
Manual MPCI-DA12-16.Dh
Chapter 5: Programming
The PCI-DA12-8/16 DACs, Timers, and Digital I/O use 40 consecutive I/O addresses. Programming
the PCI-DA12-8/16 is very straightforward as there are only three operating modes, three
range-selection switches per channel, and one unique addition. The basic operation of a
Digital-to-Analog card is to write a 12-bit value to a Digital to Analog Converter (DAC) pre-load
(outer) register where it is buffered and loaded by an update command to a DAC (inner) register.
Outputs of that register control a "ladder" network which produces the analog output. The output
voltage range is defined by settings of the range-selection switches for that channel. In C:
outport(BASE+(CH*2), (Volts*4096/10)-2048);
would output "Volts" volts to channel "ch", assuming a bipolar 5V range. For other bipolar ranges,
substitute the appropriate voltage span in place of "10" in the equation. For unipolar ranges, also
remove the "-2048".
Upon power-up, or hardware reset, the DAC registers are restricted to a safe value and the card is set
in Simultaneous Update mode. Since the pre-load register is not cleared upon power-up, but left at an
undefined value, a known value must be written to the preload registers before using a "Clear
Restrict-Output-Voltage" command.
Simultaneous Update Mode is the power-up or default mode of operation for the DAC card. When
a value is written to a DAC address the output does not change until an output update is commanded
via a read from Base Address+8. (Alternatively, a read of Base Address+A will update the DAC
registers and switch the board to Automatic Update Mode.) While in Simultaneous Update Mode, a
single read will load all DAC registers with the value waiting in the pre-load registers, causing all
outputs to be updated and changed simultaneously.
Automatic Update Mode is the configuration that changes a DAC output immediately after the
high-byte of the new value is written to the DAC address. If the card is in Simultaneous Update Mode
a read of Base Address+2 will change the card back to Automatic Update Mode without updating the
outputs. A read of Base Address+A will update all outputs simultaneously and then place the card in
Automatic Update Mode.
Timer Update Mode is similar to Simultaneous Update Mode, except that updates are issued by the
counter/timer (programmed in mode 2 or mode 4 per Chapter 6, Programming 8254). A read from
Base Address+5 will change the card from Simultaneous Update Mode to Timer Update Mode, and
a read from Base Address+6 will change it back. The counter-generated update pulse is also available
at the 40-pin header connector(OUT2) to synchronize external devices, and can generate interrupts for
synchronized loading if enabled by a read from Base Address+3. (Interrupts are disabled by a read
from Base Address+4.)
Restrict-Output-Voltage limits the output of all DAC channels. This is done by reading Base Address
+E. The previous outputs will be restored when a "Clear Restrict-Output-Voltage" command is issued
by a read of Base Address+F.