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Radio Programming Interface

Building a Multi-Purpose Radio Programmer

This page describes how to build an RS-232 to TTL level converter that can be used to program quite a few different types of amateur and commercial radios and pagers.  It has been used successfully with the Kenwood TH-G71A, Yaesu FT-2600M, Yaesu FT-8800M, Motorola GM300, and Motorola Minitor III.  It will probably work just fine with similar radios, but use at your own risk.

A schematic of the programmer is below.  The circuit uses a MAX232 IC, which converts the +/- 12V RS232 signals from your computer to 0-5V TTL signals that the pager uses.  A 9V battery powers the circuit, so a 78L05 voltage regulator is used to drop the voltage down to 5 volts.  Resistor R1 and the LED just indicate that the circuit is powered, and could be removed if desired.  Resistor R2 is only needed if you want to program Motorola GM300/Radius radios or other radios that have a single communication line and require an “open collector” (the diode that goes with this resistor is in the interface cable…see here).  See the bottom of this page for troubleshooting tips.

Programmer Schematic

Parts List

Part                     Description                                    All Electronics Part # 

U1                         MAX232                                         SP232ACP
U2                         LM7805                                          7805T
C1,C2,C3,C4      10uF electrolytic capacitor   10/16VR
R1                         1k ohm resistor                            1K-1/4
R2                         10k ohm resistor                         10K-1/4
D1                          LED                                                   LED-2
P1                           DB9 Connector                           DB-9S and DB-9H
J1                           Stereo Jack                                   MJW-20 
S1                           SPST Switch                                  MTS-4PC

 I used a 3.5mm stereo jack for the connector on the “radio” side of the interface, since it’s a readily available and inexpensive 3-conductor connector.  I built the circuit on a prototyping PC board available at RadioShack.  A few pictures of the completed programmer are shown below for reference.  You'll need to build an interface cable in order to use this programmer with a radio or pager.  For some examples, see the Interface Cables page.


Inside view

Front View

Rear View

Hints and Troubleshooting

Here are a few things to check if your circuit doesn’t work the first time (it never does for me!)

· The polarity of the capacitors does matter! The curved line on the schematic is negative, the straight line is positive. C1 is the one that usually gets messed up, since the positive terminal of the capacitor goes to circuit ground. Counter intuitive, but that's the way it has to be.

· The MAX232 is a level converter that changes RS232 voltage levels to TTL voltage levels. RS232 uses +12V to represent a "0" and -12V to represent a "1". TTL levels are 0V for a "0" and +5V for a "1". So, to see if the circuit is working, measure the voltage on pin 3 of the DB9 when it is plugged into the computer. This would be the voltage between pin 3 and pin 5 (GND). If the voltage is -12V, the voltage on pin 13 of the MAX232 should also be -12V (since it's a direct connection to the DB9 pin 3), and the voltage on pin 12 should be +5V. If the voltage at pin 3 of the DB9 is +12V, the voltage on pin 12 of the MAX232 should be zero. That covers the path from the computer to the pager. To check from the pager to the computer check the voltage on pin 11 of the MAX232 with the pager connected and in programming mode. If the voltage is zero, then pin 14 on the MAX232 should be at +12V (it will probably be more like +8V, but that's good enough). If the voltage on pin 11 of the MAX232 is at 2.5V or higher, then the voltage on pin 14 of the MAX232 should be -8 to -12V.

· Here is a link to the datasheet of the MAX232, if you don't have one already http://pdfserv.maxim-ic.com/en/ds/MAX220-MAX249.pdf

· If you have an oscilloscope it makes troubleshooting much easier...you can trace the serial data through the circuit to see where it's not getting through

· One of the more common mistakes when building circuits like these are getting the TX and RX accidentally swapped somewhere in the circuit.  Be sure to check this.  Note:  The original version of the schematic on this document had the TX and RX lines swapped on the DB9 connector.  (DOH!)  If you’re using an old schematic be sure to correct this.