Analog Input (A/D)
The PA-CP12 provides 16 single-ended channels, of 12 bit, 200us
analog to digital conversion.
The signal connector - The analog input signal connector is a 37 pin D-type male connector.
A/D channel selection - The PA-CP12 is configured for 16 single ended inputs only. Writing to base+2 port selects
the channel that is to be converted.
Sample and hold -
The PA-CP12 does not provide a sample and hold circuit.
A/D conversion control -
The A/D conversion can be triggered in three ways, they are selected by programming the
command port base+3.
Trigger method 1 : S/W trigger conversion
This method of conversion is very easy to implement because the software is directly
controlling the A/D conversions. The conversion will be started
by writting any data to the S/W trigger port (base+0).
Trigger method 2 : External clock conversion
The external clock conversion consists of using an external clock or pulse (coming in from a
jumper) to trigger the A/D conversion. This gives the user
great flexibility in synchronizing the A/D with other devices.
Trigger method 3 : 8254 timer conversion
The 8254 timer conversion method uses the 8254 timer controller to generate an accurate
time pulse which triggers the start
of conversions. An option that goes with this is to gate the 8254
timer trigger signals with an external signal. This gives the user
external control over A/D conversions using the 8254 timer.
A/D conversion -
When an A/D conversion takes place, the EOC (End of conversion) line
goes high. Once the start conversion signal
is received, conversion of the voltage at the selected channel is
initiated. Once the conversion is initiated, no other start conversion
command will be accepted until the completion of the conversion.
Upon completion of the conversion, the EOC line goes low (in timer or external clock trig
mode, an interrupt will occur). Next, the least significant byte (LSB) and the most
significant byte (MSB) of the digital value for the voltage can be
read from the data registers, base+0 and base+1 respectively.
Reading conversion result -
There are two methods to read the conversion result :
Method 1 : Polling the EOC status
This method is the simplest method to do A/D conversions and read the data. However this
method ties up the CPU because one must keeping polling the EOC bit
in the status register so that one can know when an A/D conversion is completed.
Method 2 : Interrupt
This method is more efficient than polling the EOC status, because upon a completed
conversion, the PA-CP12 interrupts the CPU to read the conversion result.
In other words, the conversion is done in the background, leaving the CPU to do more important tasks.
Conversion status -
The A/D status can be read from two ports; The base+3 port contains the critical information
for the setup and condition of the A/D while the base+2 port contains information
on the selected channel.
Analog Output (D/A)
The PA-CP12 provides one voltage output channel with 12 bit 20us
digital to analog conversion.
Data register -
The PA-CP12 uses 2 data registers or ports. The ports
are base+4 and base+5.
Using 2 ports enables the user to send a 12-bit word to the D/A
converter using an 8-bit bus without getting glitches on the output.
Here is an example of the data register scheme. Say a user wishes to
convert the digital value from 1AFH to 1B0H to an
analog value. Using the standard approach of sending 8 bits at a
time would result in a glitch on the output at these digital values--
1AFH-1A0H-1B0H (sending LSB first, then MSB) or 1AFH-1BFH-1B0H (sending
MSB first, then LSB).
However, using the PA-CP12, one would write the 0H (4 least significant bits) to the
temporary register (base+4). Next
the user would write 1BH (8 most significant bits) to the output port
(base+5). Upon writing the MSB to base+5, the data in the temporary
LSB register is automatically sent to the output, thus, both MSB and
LSB arrive at the D/A simultaneously. (i.e. no glitch)
D/A conversion -
D/A conversion is accomplished using a 12 bit D/A conversion chip. The 12-bit words are
sent from the data bus and a
temporary register simultaneously. This enables a 12-bit word to be
sent from an 8-bit data bus without getting odd glithes on the output.
The signal connector -
The connector for the D/A output signal is the same 37 pin male D-type connector used for
the A/D. The A/D and the D/A share the same connector.
Digital I/O
The PA-CP12 provides 16 lines of digital I/O; 8-bit digital output
and 8-bit digital input.
Digital output -
Digital output consists of one 8-bit dedicated output path. To write data out of this path a user
simply writes to the I/O ports; base+8 is the output port for CON1.
Digital input -
Digital input consists of one 8-bit dedicated input path. To read data from this path a user
simply reads from the I/O ports; base+8 is the input port for CON2.
The digital I/O connectors -
There are two connectors for the digital I/O. Each consists of a 20 pin box header.
Connector 1 (CON1) is output and connector 2 (CON2) is input.
| 
