Tracker-2800-2800S.pdf - 第76页

76 4. Use the red test lead from the Tracker 2800's Signal j ack. Probe the enable input pins of the IC and view their signatures on the signature displa y. For this ex ample, the enable pins of the 74LS245 are pin …

Digital Integrated Circuits

Before we examine the analog signatures of an IC, let's study the block diagram of a 74LS245 octal bi-

directional bus buffer to introduce some basic concepts. This IC is a member of the low power

Schottky transistor-transistor logic family (LSTTL). Examine the block diagram for this chip below.

You will see that there are only four different kinds of circuits on this chip.

Figure 5-1. Digital IC 74LS245 Block Diagram.

 Circuit 1 - Pins 2 through 9 and 11 through 18 are all the same function. Each pin is connected to

both an input and an output of a buffer.

 Circuit 2 - Pins 1 and 19, although they have different names, are both enables and are inputs to

AND gates.

 Circuit 3 - Power supply ground input, pin 10.

 Circuit 4 - Power supply V

input, pin 20.

Each circuit type will produce a different analog signature. Because there are only four types of circuits

on the chip, there will be only four unique analog signatures when out of circuit.

Signatures of a Digital IC

Do the following to display the analog signatures of a digital IC (out of circuit):

1. Select the 5V, 10K, 200Hz Range.

2. Place or clip the black test lead from the Tracker 2800's Common jack to the IC's ground pin. For

this example, the ground pin of the 74LS245 is pin 10.

3. Use the red test lead from the Tracker 2800's Channel A jack. Probe each pin of the IC and view its

signature on Tracker 2800's signature display. For this example, pins 2 to 9 and 11 to 18 are all

buffer circuits so they will have identical signatures. (Note: This is only for ICs out of circuit.)

4. Use the red test lead from the Tracker 2800's Signal jack. Probe the enable input pins of the IC and

view their signatures on the signature display. For this example, the enable pins of the 74LS245 are

pin 1 and 19 and will have the same signatures. (Note: This is only for ICs out of circuit.)

5. Change the range to 10V, 100, 200Hz. Use the red test lead from the Tracker 2800's Signal jack.

Probe the power supply V

input pin and view its signature on the signature display. For this

example, the V

pin of the 74LS245 is pin 20.

Buffer pins Enable pins V

Power pin

20V, 10K, 200Hz 20V, 10K, 200Hz 10V, 100, 200Hz.

Figure 5-2. Signatures of a Digital IC, 74LS245. Ground Pin to Test Common

Compare these signatures with other signatures of discrete components such as transistors and diodes.

Note that there are quite a few similarities here.

Signatures for Different Logic Chip Families

There are a wide variety of logic circuit families. Each has its special functions, advantages, and

limitations. They range from TTL and its variations (F, LS, S, etc.), emitter-coupled logic (ECL), to

the complementary metal oxide semiconductor (CMOS) and its variations (C, HC, HCT, VC, etc.).

Comparing Two TTL Logic Families

Although the logic function is the same, there are differences in the circuitry of each logic family.

These differences can be readily seen in their signatures

using the Tracker 2800.

We will illustrate these concepts with the following example of

two hex inverters, a 7404 and a 74LS04 from different logic

families. From the logic diagram below, you can see that they

have the same logic functions and pin order. The difference is that

the LS chip uses Schottky transistors in its internal

construction for increased performance and reduced power

consumption. Note that there are only four types of circuit

connections and therefore only four signatures on this

chip: inverter inputs, inverter outputs, V

and ground.

Figure 5-3. Diagram of 7404 & 74LS04.

Pin 1 input Pin 2 output Pin 14 power

20V, 10K, 200Hz 20V, 10K, 200Hz 10V, 100, 200Hz

Figure 5-4. Signatures of a 7404 Hex Inverter.

Pin 1 input Pin 2 output Pin 14 power

20V, 10K, 200Hz 20V, 10K, 200Hz 10V, 100 200Hz

Figure 5-5. Signatures of a 74LS04 Hex Inverter.

Note the differences between the two logic families. They have the same logic function but different

construction, therefore different signatures. To test one of these chips without another reference chip

available just compare each input's signature with the other five inputs. Similarly, compare each

output's signature with the other five outputs.