All IC chips have signatures on their nodes, usually on the pins. They are denoted by a hexadecimal code. For example, H3U9 may be a pin 6 signature reading for a certain IC. The signature analyzer is a trouble shooting device that tells the user whether these nodes are functioning properly. It starts off by providing a go, no-go test by reducing the complex data streams to a verifiable four digit signature. If the signature obtained matches the expected signature in the manufacturer's service manuals, the node is functioning properly. If a signature is found which does not agree with the one documented, the signal path is then traced back until a correct signature is found. This localizes the fault. Once the faulty node is found, the bad component on that node becomes apparent. The signatures on a signature analyzer are right or wrong, they are not almost-right. Like your own signature, it is very unique and must comply when writing cheques or signing documentation. The chance of obtaining a correct signature when a fault occurs is almost non-existent. The process of signature analysis is a procedure which requires little understanding on the operator's part of the circuit being tested. With its relative ease of use, unskilled personel can troubleshoot a large number of systems fairly quickly. It is a cost effective means for troubleshooting microprocessor based systems in a high volume environment.

The Hewlett-Packard 50004A is a typical signature analyzer. This instrument is relatively simple. By looking at the signature analyzer diagram, there are two sets of probes: a Data probe for sampling the individual nodes and a control pod for controlling the data capture process. The Data probe has a lamp towards the tip, which states the value of the node being tested. For example, if the lamp is on, that designates a logic high. If the lamp is off, that signifies a logic low. If the lamp is dim, there may be an open circuit or a bad level. The other probe appears on a flat card control pod and consists of Start, Stop, Clock and Ground.

With these probes combined together, they take signals required by the signature analyzer to generate a signature. The connection points for these inputs must be specified in the service documentation for the product under test.

The panel on the front of the signature analyzer contains buttons for controling the operating parameters and a four-digit seven-segment readout for displaying the signature. The most important buttons are those labelled START, STOP and CLOCK. They are used to select trigger modes of rising edge or falling edge on the Start, Stop, and Clock input probes. By pushing the button 'in', one will get a falling edge trigger, while leaving it ‘out’, will give a rising edge trigger.

There are three main aspects to the signature analysis process:

• Stimulus
• Data compression
• Data capture

Stimulus is used to create state change activity on the system nodes. This activity is essential for the signature analysis process. One type is called free running. It is a hardware-based stimulus which allows the microprocessor to cycle through its entire addressing range, allowing the operator to test and troubleshoot the bus system, address decode logic, and ROM: the "kernel" of the system. Free running is important since it requires only the microprocessor IC to be functional in order for it to run. With it we can debug enough of the system for it to run sophisticated circuit stimulus.

During data capture, the signature analyzer reads in the bit-stream information being generated by the stimulus. The period when the analyzer is reading data is known as the measurement window, or gate. There are three main signals used for control of the data capture process: start, stop and clock. The start signals tells the analyzer to open the measurement window so that data can be read in. The stop signal closes the windows, signifying that data capture is complete, and the signature is displayed. The clock signal synchronizes the analyzer’s sampling cirucuitry to the unit being tested. The gate is actually controlled by the clock and not by the start and stop signals themselves. The start and stop inputs are sampled on the active clock edge just as the data probe input is. Sources for the start, stop, and clock signals will also vary according to the application. Generally, these signals can be obtained from hardware such as address lines, chip selects, and read and write strobes. By setting these connections up, we can use the Data probe to check out various circuit nodes. For example, IC pins. We can compare the signature observed, with the reference list produced when the system was fully operational. Follow any incorrect signatures through the circuit until correct signatures are observed. There may correct signatures entering the IC, but incorrect signatures appearing on the output pins. Thus indicating a faulty IC. This is the basic procedure that must be followed in testing. The signature on ground line is always going to be 0000. However the signature on Vcc will be variable, it will depend on the type of test program being executed.

Data compression reduces a bit stream into the four-digit number known as the signature. It is implemented in hardware using a 16-bit shift register with linear feedback.

Remember, even the smallest circuit modification will often cause a signature list to be inaccurate for a set of test programs. A new set of reference signatures should be produced. Therefore there is always a need for accurate documentation covering circuit upgrades. By performing some of the lab exercises, signature analysis will become quite clear and easy to understand.

In a free-run mode the path between the data bus and the microprocessor is broken, this allows that all the devices such as the RAM, ROM, AND I/O ports to be read, while the address bus is incrementing without affecting the microprocessor’s function.

• Refer to Table 1. Along the address bus are a number of holes labeled A0 to A15. Place the probe in these holes and verify they agree with the table. For example, with probe in A0, the signature analyzer should read UUUU. This will ensure that the m p, IC1 and IC2 are working properly.

TABLE 1

• Probe any data bus line and note its signature. These are holes labeled D0 to D7 on the data bus. This signature is the combined product of all the devices of the m lab that are output to the data bus during the 64K-address window.
• Turn the m lab power on and off, observe that the data has changed. This is because the RAM has new data stored in it as a result of power-off cycle.
• With the probe still on the data bus line slide the input port switch. These switches are located towards the bottom of the microlab. Observe that the signature change depending on the position of the switch. D0 corresponds to the 0 switch, while D1 corresponds to the 1 switch and so on and so forth.
• Checking IC’s is almost the same. In the back of the microprocessor manual on page 396, is a list of signatures for each IC. By placing the probe on the pins of the corresponding IC, will give an appropriate signature. Feel free to check any of the IC chips and match them against those in the microprocessor manual. The signature should be exactly the same as those in the manual.
• The above lab is run without any feedback, this is done to stimulate other devices in the circuit in repeatable and predictable manner.

The signature analysis (SA) test loop is a program permanently stored in the m lab’s ROM mainly to provide a controlled stimulus for signature analysis. The SA switch is connected to an interrupt line on the microprocessor. This test mode also turns all the display segments and output port LED’s on. They indicate the m lab is running the SA test loop program. This loop provides stimulus signals to devices that the microprocessor talks to, for example, the output port, and display port.

In the above experiments we have learned how to minimize and isolate the fault location, incorporating a free-run mode and SA test loop program. To perform the task we only require the power supply, the test switch and the circuit directly connected to the microprocessor control pin to be functioning. Once this system is verified the SA test loop program is used to test some of the devices that can not be tested by the free-run mode.

1. The signature received from a signature analyzer should exactly match the one from a manufacturer’s service manual. True or False?
2. What are the four inputs on the control pod?
3. On the front panel, pushing in the start button will result in a positive edge trigger. True or False?
4. What are the three main aspects to the signature analysis process?
5. The signature on Vcc will always be the same value. True or False?
6. What are the two types of probes?
7. Signatures can:

1. Provide information about the nature of the fault.
2. Tell whether a node is operating correctly or not.
3. Verify the test set-up.
4. Do all of the above.

1. __________________ reduces a bit stream into a four-digit number known as the signature.
2. To find a fault, a signal path must be traced back until a correct signature is found. True or False?
3. Why is the signature analyzer easy to use?

1. It requires little understanding on the operator’s part of the circuit being tested.
2. Unskilled personnel can troubleshoot a large number of systems fairly quickly.
3. It is a cost-effective means for troubleshooting microprocessor-based systems in a high volume environment.
4. All of the above.

1. True.
2. Start, Stop, Clock, and Ground.
3. False.
4. Stimulus, data compression, and data capture.
5. False.
6. The data probe and the control probe pod.
7. D. All of the above.
8. Data compression.
9. True.
10. D. All of the above.