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The Maxon SD-125 is an FM data transceiver, available in both VHF and UHF versions. Now and then they become available on the surplus radio market.
The programming software is freely available from Maxon (see link at end of this page), however the specifications for the programming interface between the PC and the SD-125 are not.
This project is a home-brewed programming interface for the SD-125, that was produced by reverse engineering the SD-125. It has not been (and is unlikely to be) approved by Maxon, and while I have successfully used it to program several SD-125's, there is no guarantee that it will not damage your hardware, or that it will even work!
The Maxon SD-125 enters programming mode, if upon application of the 13.8 Vdc power supply (pin 5 DB9), it finds that PTT (pin 3 DB9) is pulled down to ground and the Modulation_input (pin 1 DB9) is pulled high.
In programming mode the SD-125 communicates with the PC serial port at 2400 bps, 8 data bits/character, no parity, and 1 stop bit.
Shortly after entering programming mode, the SD-125 sends the following data to the PC:
Programming EEPROM Data
+ =Up 5, - =Down 5, N =Next, / =Addrs $50
Type 2 bytes then press ` to save
00 ED0C
If you are really keen, you can use a terminal emulator to program the SD-125's EEPROM directly in hex, or you can download and install the official programming software, which is very easy to use.
To avoid unintentional transmissions, connect the programming interface to the PC serial port and to the SD-125's DB9 connector and start the programming software (or terminal emulator) BEFORE you connect the 13.8 Vdc power supply. Make sure that you disconnect the power supply before you close the software or disconnect the programming interface. If by chance you do transmit with this programming interface connected, you may burn out D2 (1n4001), the reverse protection diode. Also, if enough RF is coupled into the programming interface and rectified by D1 (1n4148) then the PTT line will be held low by Q1 and the system will "get stuck" in TX mode.
Q1 is a small signal NPN transistor - I used a BC337, but a 2n2222, PN100, 2n3904, or what have you should work ok.
D1 is a small signal diode (a 1n4148, but a 1n914 will work just as well), it's purpose is to protect the base of Q1 from the -12 V level that can be present on the PC serial port handshaking lines under certain conditions.
R2 is a suplementary pullup. The SD-125's BUSY output (pin 6 DB9) is an open collector with a resistive pullup, unfortunately the resistor installed inside the SD-125 by Maxon did not allow enough current to flow
as the pullup resistor inside the SD-125 was too large (and my serial cable and port capacitance too high) resulting in corrupted data. The solution was simply to install a suplementary pullup resistor to decrease the risetime of a low to high transition on this pin.
Programming interface schematic
Maxon SD-125 information
Maxon SD-125 programming software
This page last updated 18th December 2011