FM1200 mobile radio.
There are several different types of this mobile radio. The modification kit has been tested for B0, K1, STM22 and STM23 versions. It is possible to use VCO steps of 5, 6.25, 10, 12.5 and 25kHz, with the new control software the radio has 100 memory channels with name of max. 23 characters which are programmable with the keypad of the radio. For digital types, a hardware modification is needed for generating and receiving tones for sel-call. The controller used for the internal (trunkin) modem, is now used for generating DTMF, CTCSS and gererating or receiving single tones as sel-call and 1750Hz burst. Some wires on the printed circuit board for the modem, must be cut. These are outputs of the controller, the controller gives the tones to transmit on these outputs. The interrupt input of the controller is used for measuring frequency of a tone from the receiver. The signal of the receiver is amplified by an opamp in open-loop, this is done by removing the 22k resistor, the received audio results in a square ware for the interrupt input of the controller. The controller measures the period time of the falling edges on the input. If the period is the same for eight times within a certain range, the period will be send to the main controller which calculates the frequency. Most functions are inplemented in the main controller, the tone controller software has to be very fast to get a reasonnable resolution. Another duty of the tone controller is transmitting CTCSS tones, this requires speed. Because the CTCSS tone is mixed with the audio signal, the tone should stay below the 250Hz and may not give any harmonics. To generate an as good as possible sine wave, the controller has to produce as many samples as possible. An DA converter is a good solution, but the controller hasn’t enough outputs and connecting an ADC is a lot of soldering work and wires. The tone controller generates PWM pulses, the modulation is the sine wave for CTCSS. The frequency of the square wave is a several times higher than the sine wave itself, this makes filtering very easy. The controller runs a program of 17 lines which uses a look-up table for setting/resetting the output. Every CTCSS tone has another look-up table.
For this modification are 2 to 4 ROM chips nescesary. The EPROM in the display unit has to be replaced, this is a small program which does nothing else than printing the data from the FM1200 to the display and sending the last pressed key to the FM1200. This software has to be programmed in a 27C128, a 27C256 or a 27C512 EPROM. De EPROM of the main controller must be replaced and the 24C16 can be replaced by a 24C64 or a 24LC128 to use longer names an tone-codes.
The last EPROM is for the tone processor, this is only nescesary if tones are used. It is the SMD EPROM behind the plastic cap, to remove the EPROM, it is better to cut all pins to save the board. The software for this CPU has to be programmed in a 27C256 or a 27C512 EPROM in SMD socket. Smaller EPROM’s have a different pinout.
This is an example of mounting the extra components:
The SMD components at the red circles have to be removed. At the top, left, a resistor of 22k, and on the right a 1k resistor. The green squares are indicating the wires to cut. Another resistor need to be soldered to the pad of the SMD resistors
* The first startup of the software will reset the contents of the EEPROM, it is set to 70cm.
For 2m radio’s the band has to be changed in submenu ‘User’.
* One of the two communication lines between tone CPU and main CPU are not routed on CPU board version 1.
In this case, you have to make a connection between pin 17 of the main CPU and pin 14 of the tonecpu to get tones working.
* If the EPROM socket on the CPU board has 32 pin instead of 28 like an 27C512, pin 1, 2, 31 and 32 must be left open.
The EPROM must be placed as far as possible to the display unit.
There is a small description of the data stored in the EEPROM available here.
Some FM1x00 owners are using a VOX unit in the car, the VOX circuit can be use as an hold circuit for the PTT key by soldering 1 resistor of 12k between the two other pins of this IC:
Please refer to the usermanual for functions and how to use them, an English version is available at the bottom of the page.
27C256 PLCC package or
27C512 PLCC package