To use apparatus intelligently it helps to understand what is going on inside; the discussion below focuses on giving some insight into what occurs when you do an analog conversion. As with most things, such discussion has many layers of increasing depth and detail. This discussion will go only one veneer down.

The analog to digital conversion is done on the John Bell Engineering Universal I/O board by an ADC 0817 IC which is connected to the address and data busses and to the RD (read) and WR (write) wires of the IBM-PC (Figure D.I). Port addresses 812–819 (assuming a base address of 800 decimal) are devoted to operation of the ADC.

There are basically three operations which can be done. Refer to Figure D. 1 in the following. If a byte is written to any of the port addresses 812–815 (e.g., Port[812]: = n;), the address is decoded by the card circuitry and the CS4 wire is activated. The WR wire is also active since this is a write operation. Thus the data wires (D0–3) at positions ABCD on the multiplexer are activated. These four wires select by means of a binary code, the position of the switch so that one of 16 analog input channels is connected to the input of the converter section. Thus the lower four binary digits of the byte (n) are used to set the multiplexer channel number. Just after the multiplexer is switched, the converter is started since the WR and CS4 wires are also connected to the Start input of the converter section.