Design Ideas: June 6, 1996
Figure 1 is a simple cost-effective waveform generator using PC control. You calculate the digital image of the desired waveform and store it in the 32k X 8-bit RAM (62256) through the 74HCT245 data buffer and the 16-bit local-address bus generator built with the 74LS393 binary counters. The 74LS244, acting like a selection switch, selects the clock input of the binary counter. For the initial loading of the digital image to the RAM, port address 300 (hex) connects to the clock input of the counter through the 74LS244. For the loading operation, the low-state Q output of the 74LS74 flip-flop enables the 74LS244.
For unloading the digital-image pattern from the RAM to the D/A converter, the PC clock becomes the clock of the binary counter. For the unloading operation, the flip-flop's Q output enables the second half of the 74LS244. Port address 301 (hex) triggers the flip-flop and starts the generation of the waveform by enabling the D/A converter. In this design, the PC need only intervene for filling the digital-image pattern in the RAM. The local-address bus, clocked by the PC clock, generates the required waveform independently of the PC. The bus drives the RAM and activates the D/A converter for the conversion of the digital-image pattern to the desired analog waveform.
You can load the digital-image pattern for any type of waveform by calling the port address 300 (hex) with the appropriate data. For example, to obtain a triangular waveform, load 00 into the 0th location of the RAM and increment the data in steps of one to FF in the subsequent RAM locations, then decrement the data in steps of one from FF to 00. (DI #1883)