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Keypad Connections

One of the tasks in using the M/P is finding a suitable keypad. There are many sources for fine new keypads, including eBay on the internet. The apparent problem with purchasing a keypad is that even new ones do not come with schematic drawings. How then do you determine which lead is which??

    The M/P controller requires a 4x4 keypad. These keypads have 16 individual SPST push button switches arranged in a 4x4 switching matrix. Pushing a switch makes a momentary closure between one (and only one) lead in the row and a corresponding lead in the column groups. This momentary contact closure is all that is required to 'trigger' the interrupt processing logic within the M/P to 'examine' the key.

    If your keypad comes with a schematic, you're all set to connect it. If there is no schematic and if you feel adventurous, you may be able to take it apart to 'trace' the P/C wiring. However, opening up a keypad (especially one that has been glued together) is often not an easy task and - done improperly - may permanently damage the unit.

    However, there's a much easier way to determine 'what's what' by simply using your meter. Here are the steps:

If your keypad does not have wire-wrap pins already installed, make up an 8 pin terminal strip and solder it on.

Measure the length of the wires needed to connect the keypad to the M/P within your enclosure, leaving enough slack for any subsequent servicing.

Cut (8) 30 gauge leads and connect one to each of the pins.

Strip the other ends of the wires back and lay the unit onto your workbench.

Set up your meter to read resistance (1K scale or higher), and connect it to any two leads.

Then, slowly depress each of the 16 keys watching for a deflection on your meter. Since these contacts - when closed will not produce an exact 0 ohm 'short' (30 ohms is more like it), you'll have to watch closely. If you are unable to get a 'hit' after trying all the 16 individual keys, this means that the 2 leads you have selected are both in either the row or column group.

Wrap a piece of solder around both of the leads for identification purposes, and disconnect either one of the leads. Leave the remaining lead hooked to your meter. This is your anchor lead.

Connect your meter to another lead and slowly push each of the 16 keys, watching for a deflection. If you see no deflections, then group the newly connected lead together (under the solder) - but don't disconnect the 'anchor' lead'. So far, you have identified 3 leads that are all in either the row or column group.

If, however, you see a deflection on your meter, this means that you have identified a connection between a row and a column. Note the key that was depressed to produce this deflection. If, for example, you see a deflection when you push the 4 key, this means that you have a connection between Row 4 and Column 1. But which is which?

Tag the lead you had just connected as 4 and remove the meter from it. Do not disconnect your anchor lead yet.

Select another lead and connect it to your meter.

Push the remaining buttons (except 4) and watch for a deflection.

If you see no deflections, disconnect the lead and place it under the solder as you have now identified another common row or column connection.

If you do see a deflection, it will be on either key 1, 7 or * - or on 5, 6, or B.

If on 1, 7 or #, this means that your anchor lead is definitely Row 2, and that the other lead is Column 1. If on 5, 6 or B, this means that your anchor lead is Column 1, and that the other lead is Row 2. You also know that the leads grouped together under the solder are either all row or column leads based upon the categorization of your anchor lead.

Accordingly, disconnect the meter from both leads and connect them to the KEYS terminal strip on the M/P. Now you have identified and connected 2 of the 8 leads.

Select another one of the leads grouped under the solder and connect your meter to it. This is your next anchor lead

Hook your meter to another ungrouped lead and push each of the remaining 12 buttons, watching for a deflection.

If your original anchor lead turned out to be Column 1 and if you see a 'hit' when pushing the 8 key, this means that your new anchor lead is Column 2, and the other lead is Row 3. Disconnect them from the meter and connect them accordingly.

In a similar fashion, identify and connect the remaining leads.

As an example, a nice 16 button keypad was purchased on eBay for $4.00 with free shipping. It came from China. Here's a picture of the front, and here's a picture of the back.

Using the above method, the leads were determined to be (left to right across the top, looking at the front of the keypad):

C4 - R1 - R2 - R3 - R4- C1 - C2 - C3

DISCLAIMER - - If you follow the steps outlined herein, you do so at your own risk. I cannot, nor will not, be responsible for any possible damage to radio equipment, personal property, to yourself or to others caused by modifications that you may make to the radio as a result of your reading this.

The M/P controls TRANSMITTING as well as receiving on many frequencies, suitable for a wide range of HF, VHF and UHF needs. In the USA, TRANSMIT operation requires a license issued by the FCC for the class of operation intended. Amateur radio licensees must maintain strict control over their equipment, preventing unlicensed operation within the amateur bands, or outside of them.

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