One component of a PLC control circuit is the thumbwheel switch; speedy testing of these devices will reduce downtime.

Thumbwheel switches, and their associated wiring, can be sources of trouble when there's a breakdown in a manufacturing process line. By understanding how these devices work and how to test them, you can decrease the time needed to troubleshoot the problem and keep downtime to a minimum.

Types of thumbwheels

Numerous examples of thumbwheel switch applications can be found in the typical high-tech manufacturing world. These devices perform numerous control and coding functions, such as inputting digital signals into a programmable logic controller (PLC) or other type of electronic control.

There are three basic types of thumbwheel switches: octal, binary coded decimal (BCD), and hexadecimal. In the octal system, eight decimal numbers (0 through 7) are translated into binary code by four contacts that are built into the thumbwheel switch. In the BCD, 10 decimal numbers (0 through 9) are represented. In hexadecimal, a total of 16 numbers and letters (0 through 9, and A through F) are represented by the four thumbwheel switch contacts.

Contact positions

It's important that you know how the positions of the switch contact (open or closed) correspond to the decimal number (arrived at by adding the contact numbers) or letter that appears on the thumbwheel switch window. A handy guide for this is a truth table, a copy of which is shown in Fig. 1. Fig. 2 (on page 22) shows the arrangement of the contacts of a thumbwheel switch, with the four contacts numbered 8, 4, 2, and 1. A common terminal is also provided.

When dialing in the decimal number "3" on the thumbwheel, you can see from the truth table that Contacts 2 and 1 are closed (noted by "1" in Contact 2 and Contact 1 columns), while Contacts 8 and 4 are open (noted by "0" in Contact 8 and Contact 4 columns). The decimal number "9" in BCD and hexadecimal code is represented by Contacts 8 and 1 being closed (noted by "1") and Contacts 4 and 2 being open (noted by "0").

In addition to the four contacts (8, 4, 2, and 1), thumbwheel switches can have a complementary (opposite) set of contacts. These contacts are identified by a bar over the "8," "4," "2," or "1" numeral, as shown in the righthand portion of the middle thumbwheel switch in Fig. 2. These contacts are referred to as "NOT" contacts.

When Contact 8 is closed on the regular portion of the switch, Contact 8-NOT is open; when Contact 8 is open, Contact 8-NOT is closed. The same is true for the other contacts.

Another variation on the basic thumbwheel switch uses diodes in series with the thumbwheel switches (as shown in bottom thumbwheel in Fig. 2). Whether diodes are used or not, and whether the cathodes or anodes are connected to the switches is determined by system design. Design also determines the type of thumbwheel switch used; whether provisions are made for connecting to ribbon cable; or whether the wires are soldered to the terminals.

Thumbwheel tester

When a thumbwheel switch or its associated wiring is suspected of being defective, you can use an ohmmeter to check the binary codes of the unit. If diodes are part of the switch, then the ohmmeter also can be used to check these out. While this will tell you if there is a problem or not, it's very time-consuming. To speed this troubleshooting along, you should use a thumbwheel switch tester, which can be easily constructed. A schematic diagram for one is shown in Fig. 3. It can be mounted in any type of enclosure.

The tester is a continuity checker; thus, the circuit on which it's used must be deenergized. Since the open-circuit voltage of the tester is only 9V and the test current is only about 0.8mA, it's safe to use the tester on various types of electronic equipment.

To test a thumbwheel switch that does not have signal diodes in series with it, either the red or green LEDs of the tester can be used. As an example here, let's select the red LEDs. First, set Switch SW1 on the tester to the "RED" position and Switch SW2 to the "NEG" position. Next, connect the test leads from the test lead jacks to the corresponding terminals of the thumbwheel switch. Now, move the thumbwheel from digit to digit, noting how the binary code (indicated by which LED is lighted) matches that shown on the truth table. Each decimal digit on the thumbwheel should have a specific binary code. For example, Decimal 3 should light the LEDs marked "1" and "2" and LEDs marked "4" and "8" should be off; and so on.

If the binary code does not match the decimal value, then the switch is defective. If the binary code matches, then the defect is in the wiring. When switching from one digit to the next, you also should note if the LEDs flash when the thumbwheel is between digits; if they do, the switch is defective.

To test thumbwheel switches that have diodes in series, connect the test leads as before. Next, place SW1 in the "RED" position and SW2 in the "NEG" position. Now, step the thumbwheel switch and watch the LEDs. If none lights up, it means that the series diodes are reverse biased and won't allow current to pass. If the binary code is displayed (the LEDs light), then the diodes are forward biased, which allows current to flow.

Suppose the former condition exists (no LEDs lit). Then, you should move Switch SW1 to the "GREEN" position and Switch SW2 to the "POS" position, and step the thumbwheel switches to check the binary code. By moving the switches to the "GREEN" and "POS" position, you reverse bias the diodes and allow stepping the thumbwheels through all their positions without turning on any of the LEDs.

When reverse biased, a lighted LED on the tester indicates that a problem exists. For instance, assume that the thumbwheel is stepped to "4" and the LEDs indicate "0100." This would be a sign that the diode in series with the "4" position of the switch is shorted.

On the other hand, in the forward biased condition, an open diode can be detected by the indication of an improper binary code. For instance, if decimal digit "7" is turned to, the tester should indicate "0111." If the actual reading is "0110," then the diode in series with Switch Terminal 1 may be open. To confirm this, you have to disconnect the diode and check if the proper code is exhibited with the diode out of the circuit.