The software to operate the Active Load Unit is written in Excel Visual Basic along with some cell formulas to convert the desired current into a serial data stream. This data is then sequentually sent over the parallel port to the control board.
A free drive that allows NT/2000 machines to talk to the parallel port directly needs to be installed in order for the Basic scripts to function.
This note describes the parallel port pinout for an IBM PC.
The parallel port has three basic registers
The remaining pins are ground
The DTL2A-LC data sheet describes how to program the desired current level. A page in the worksheet builds the serial message from the user's input page. The bit pattern is put together and written to the port as soon as the cell loses focus or the user forces an entry.
The interface between the PC and the Datel device is a simple encoder/decoder implimented with an FPGA. The address of one of the 16 devices is output on the PC port selecting that device. Next the control pins are sequenced and the data written and latched. The process is repeated as needed.
If different control/loading features are required they can be implimented by reprogramming the FPGA or by changing the spreadsheet.
The Datel device has a Fault status pin that is active low when the voltage drop across the device is below 0.6V, this would indicate an improper bias voltage for correct operation. This signal is encoded and read back by the PC for visual display.