The following two frequency writer circuit diagrams use MAX232, a special chip for computer serial port level conversion, which has the advantages of good compatibility, good stability, simple peripheral circuits and easy production.
In fact, when writing frequency to the intercom, the function of the frequency writer is to convert the level. In general, it is to convert the information sent by the computer into information that the intercom can read, just like a translation.
Figure 1: This is a circuit that writes frequency to some hand platforms and car platforms that need to be connected with 4 lines. For example, TK3107 should connect TXD (transmit), RXD (receive), GEN (ground), + 5V, these four lines of machines. 
Figure 2: This is a machine that needs to be connected to the three lines of DATA (bus), GEN (ground), and + 5V, such as GP88 and GP88S. (Among them, transmitting and receiving are switched by a diode or a transistor) 
First of all, you need to purchase the following accessories:
MAX232 chip is about 6 yuan
18-pin IC holder for about 4 cents
4 electrolytic capacitors of 50 volts and 10 microfarads, each with about 1 cent
One RS232 female head (inserted into the COM port of the computer) about 2.5 yuan per set (with housing)
1N4148 diode one about 1 hair one
A USB extension cable (1.5 meters or longer) about 2 yuan a small piece of a universal board (half a slap is enough) about 1 yuan cheap accessories can buy a few more, in case it is lost, broken or cat cat Now, the dog gnawed and so on, hehe! !
(The above prices are local retail prices, which may be different from your local ones, but they are not very different. Be careful of JS.)
At the same time, you also need to prepare some tools such as soldering iron, solder, rosin, screwdriver and other tools and materials.
For the pins of chips and other accessories, please see the following pictures. As long as you carefully control the soldering, you can use it without debugging (and no debugging) if you make no mistakes.
MAX232 chip
IN4148 positive and negative discrimination: 
Judging the positive and negative of the capacitor:
The RS232 connector looks like this, and its pin arrangement: 
Production process:
Take the USB extension cable, cut off the female head (that is, the one that cannot be inserted into the USB port of the computer), and then cut it off about 20 cm away from the male head (of course the remaining one) Section 4 core wire is used as a wire connecting the frequency writer and RS232 plug.
The next step is to first solder the IC holder to the universal board, then solder the four peripheral capacitors and diodes, connect the wires, and finally insert the MAX232 chip. The reason why I consider using the IC socket here is to prevent the wrong position and cause many times of unsoldering the IC to cause damage, and the chip can be removed in the future for other circuits. If you don't think you need to do this, you can also save the IC socket and solder the chip directly to the circuit board.
If you use the IC holder, carefully see if there is a small semi-circular notch on the end of the seat, this corresponds to the notch on the chip (note the place where the red circle in the picture of MAX232 above me). It is used to mark the direction, in case the chip is inserted backwards, just pay attention when you do it.
If you really do n’t know how to solder, find a friend to do it. After all, it is a very simple circuit, which is easy to deal with.
It should also be noted that the + 5V power supply is taken from the USB port, that is, using the male cable that is about 20 cm long and the red wire inside is connected to the + 5V of the frequency writer. Don't connect the other UBS ports, and don't let them short circuit, just wrap them, otherwise the USB port is burned, don't blame me.
Paste a PCB diagram for everyone to refer to how the circuit goes.
Note that this picture is a PCB picture, which is the soldering side of the circuit board, and pay attention to the pins on the top, don't make a mistake. 
The final product picture: 
Figure with power supply 
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