Mini logic analyzer.

AFTER YOU HAVE COMPLETED YOUR latest logic circuit design, you will probably want to build a prototype to test its operation. If the circuit is not complex, a common logic probe is all that's needed to troubleshoot it. However, if the circuit has multiple signals that must be checked for proper time phasing, or if it requires one or more complex driving signals to exercise its operation, then you need a more powerful testing tool.

Professionals use a logic analyzer and function generator to make these tests. Experimenters, hobbyists, and those on a tight budget will want to build the PC Mini Logic Analyzer. It costs only about $30 to build, but when it is linked to an IBM-compatible computer, it is sufficiently capable for most hobby applications. The PC Mini Logic Analyzer provides up to eight driving signals (outputs) and eight inputs. Each output can be programmed with up to a 64-bit pattern. In addition to its logic analyzer function, the unit can serve double duty as a digital integrated circuit (IC) tester.

The analyzer consists of a hardware interface with applications software. The interface buffers the signals that are sent from the computer's parallel port to the circuit to be tested. It also buffers the signals that are returned from the circuit to the computer, and shifts their voltage levels so that they are compatible with the PC's logic levels. The interface obtains its power from the circuit under test, so it will always recognize the proper logic levels: TTL at 5 volts, or CMOS from 3 to 15 volts. The interface can be connected directly to any parallel port on your PC.

The software displays 64 bits of the eight outputs and eight inputs simultaneously. It allows full on-screen programming of the outputs. Scan time (the time to process the 64 bits) can be adjusted from a high of about 100 bits/second on an average PC to a low of one bit every 10 seconds. The slower speeds allow you to single step through a circuit and observe how each output bit affects the prototype circuit's operation.

Best of all, the interface circuit is composed of common components, all available from most electronic component suppliers. Although a PC board layout is provided here, it is not essential for proper operation; the circuit can also be constructed on a solderless breadboard.

The hardware interface will be described first, followed by the software. Then, an actual logic analyzer application will be described, followed by an example of how to use the analyzer as a digital IC tester.

Theory of operation

The analyzer incorporates two basic functional blocks; a transistor buffer/inverter section, and an analog switch section that feeds voltage comparators. The transistor buffer/inverter section is shown in the schematic in Fig. 1.

Transistor Q1 in Fig. 1 is configured as a standard inverting switch. A signal greater than about 0.7 volt from the parallel port (pin 2 for Q1) causes the transistor to conduct, raising the output (O1 for QI) to about 0.3 volt. When a signal …