Optical transceiver FTDI-POF
Hi, Habr, I am a student in the specialty of lasers in info-communication systems and for the first time I worked with fiber. I was invited to participate in one project related to the transfer of data on optical fiber and I took interest in this work with interest.
Fiber is becoming stronger in our life. This is the Internet, the connection of a variety of sensors, laser devices. It is also used in the lighting of rooms.
Optical lines have a number of advantages: there are no problems with grounding, high noise immunity, secrecy (there is no electromagnetic radiation that can be overheard), lightness.
The task: to create a working prototype of a board with a serial port, whose signals are transmitted over a plastic fiber-optic communication line. Based on the programmer MBFTDI, built on the chip FT2232H. JTAG connector from the programmer had to be removed and replaced with an optical receiver and transmitter. Which was done.
articles on Habré , I was very puzzled. That's what puzzled me: with my grades "excellent" and almost 100% attendance at all classes (yes, I did not miss a chance to praise myself) I could not assume that there is a POF (Plastic Optical Fiber). This plastic fiber, which is suitable for our task.
Its dimensions are 2.2 mm in diameter. For comparison: the diameter of the standard quartz fiber is 125 microns.
Processing is almost not required. You can cut the fiber with a conventional knife and not process the end, but it will still work.
The receiver and transmitter together cost slightly more than 1000 rubles, which is less than devices for working with quartz fibers.
The minimum absorption is in the visible spectrum - 650 nm (red color)
Naturally, the transfer rates are better for quartz fibers, but POF is also good for us. Its advantages make our work easier and more convenient, and the low price of components generally pleases us.
Let's move on to the development process itself.
From beginning to end in 3 steps
To work with this fiber we need a receiver and a transmitter. The choice fell on SFH551 (black) and SFH756 (white), respectively.
To begin with, a trial model was assembled on a printed circuit board, on a cable with a length of 15-20 cm all worked, but when the entire coil was connected (about 100 m), the radiation was scattered in the fiber and did not reach the end. It seems that the LED in the transmitter is too weak. Having understood the documentation, it became clear that the transmitter needs more current. The FTDI chip on the output can not give more than 12mA, but you need at least 50mA. This problem is solved by adding a transistor to the circuitry.
Having received a working optical isolation scheme, we proceeded to the layout of the board. As a basis, of course, the MBFTDI programmer circuit was taken. Further work was carried out in the DipTrace program. I did not find a library for her with our elements and made them myself. Honestly, I used this program for the first time, but its clear interface helped me to quickly get used to it. Also in DipTrace there is an interesting 3D Preview function that shows the board with all the elements in 3D. This for me was in its own breathtaking way, as its element can also be assigned a 3D model. And, of course, I made them. The work was carried out in Compass 3D. The created models can be converted to a format suitable for DipTrace - STEP.
That's what the model turned out at the end, next to the finished board:
Having working devices on hands, the phase of researches has come. First, we checked to see if our prototype is working. Yes, everything worked perfectly: at a speed of 921600 bits per second at a distance of 100 meters, the data was successfully transmitted and received. For the reception and transmission programs PUTTY and TeraTerm were used.
Now there was a question: is it possible to transmit information via one cable? And if you can, how?
For example, you can assemble a circuit with light-dividing cubes.
Here we are already thinking about the price of cubes, and this option immediately disappears. Well, okay, we'll look for something else. And we will find! We remembered about optical splitters.
I watched on the Internet, but for some reason no one offered them for sale (or I looked badly), although it was several articles about this theme.
Well, in any case, the idea is clear to us, you can try it yourself. We will make on each cable on a bevel, approximately up to the middle. By connecting them, we get something like an X-divisor.
The notches are made with a clerical knife, no calculation of the power division is necessary, the losses at the junction are too high. But as a small experiment - it suits us, even something happened.
We also need to remember that when adding dividers to the circuit, the power reaching the receiver decreases: with the cubes we get 25%, with dividers of 50% of the initial power. This, too, does not really suit us.
The short conclusion is
Pleasant acquaintance with DipTrace. Unexpected acquaintance of POF and work with it. Getting experience (where without it) and just a joy for the work done. Unfortunately, the question remained about the use of one cable for transmission in both directions.
A library of two AVAGO elements can be found at GitHub .
I hope that you liked the article, if you have any advice, I will be glad to read them.
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