Build the Digilyzer. (digital analyzer)

Build the Digilyzer: It performs some digital analyzer functions, but it costs a lot less--and it fits in the palm of your hand.

A DIGITAL ANALYZER IS AN EXtremely useful instrument for troubleshooting digital circuitry. Unfortunately, digital analyzers are usually priced beyond most hobbyists' budgets. However, the Digilizer, the subject of this article, can perform some of the functions of a digital analyzer. And the best thing about the Digilyzer is that you can build it for less than $50--a lot less than the purchase price of a factory-made analyzer.

The Digilyzer monitors the logic levels at eight of its inputs (called the test inputs), and when they match a user-set bit pattern, it latches the binary data present at its other eight inputs (called the data inputs). Once data has been latched, it is displayed on the unit by eight tri-colored LEDs. Digilyzers are end-stackable so that multiple units can monitor 16, 24, or 32-bit wide buses--this makes them quite versatile.

One of the most popular applications for the Digilyzer is monitoring the input and output from an integrated circuit to verify its operation. However, the Digilyzer performs many other complex tasks such as monitoring any memory location on a bus.

To use the Digilyzer you start by connecting the test inputs to the address bus, connect the data inputs to the data bus, and set the bit pattern for the address you want to observe. When the Digilyzer next encounters that address, it latches onto the data.

The Digilyzer also offers other options that make it especially suitable for troubleshooting. For example, you don't have to define all the bits that make up the bit pattern. Some can be left in a "don't care" state. This is useful for checking computers with faulty buses that have an intermittent line. It also permits you to observe what is occurring over a wide range of addresses.

The Digilyzer has two modes of operation: latch and free run. In the latch mode, the unit latches onto data when the test inputs match the user-set bit pattern, and it ignores subsequent matches. In the free-run mode, the latched data is updated each time there is a match.

The Digilizer is fitted with a BNC output that can trigger an oscilloscope when it detects a match. That feature allows the oscilloscope to display the serial data produced by a device such as an RS-232C port when its control and data lines are at user-specified values. This feature is particularly useful for testing parallel-to-serial converters, checking the protocol of serial ports (you'll be able to "see" the stop, parity, and data bits), and determining the handshaking lines being used by a device.

The match detector

The schematic shown in Fig. 1 shows three main sections: a match detector, a match-signal processor, and a data buffer that performs double duty as a display driver. An 8-bit identity comparator, IC1, accepts two 8-bit words (denoted A and B) and compares them.

If each bit of the two words match, the output pin 19 goes low; if any corresponding bits in the words don't match, the output remains high. Moreover, when a match is found, the output will go low only if the enable input (EN pin 1) is low. When the enable input is high, the output remains high.

The bits that form the A word input to IC1 are supplied by TEST inputs TP1 to TP8. Each of the eight bits for the B word can be user set by switches S1 to S8. Each of the SPDT center-off switches can be set in one of three positions:

* the low position that ties a B input to ground

* the high position that allows a B input to float high through a pull-up resistor

* the "don't care" position that ties a B input to its corresponding A input, ensuring a match regardless of the A bit's value.

The ENABLE input functions as part of an optional clock input (TP9). If used, it gives IC1 the ability to sense the clock of the device-under-test (DUT). …