Application of Photoelectric Coupler in Parallel Port Long Line Transmission

1 Introduction

Optocoupler (hereinafter referred to as optocoupler) is a photoelectric device composed of a light-emitting device and a photosensitive device. It can realize the conversion of electrical → optical → electrical signals, and the input signal is isolated from the output signal. At present, most optocouplers use gallium arsenide infrared light-emitting diodes for input, and silicon photodiodes, silicon phototransistors and light-triggered thyristors for output. Because the peak wavelength of gallium arsenide infrared light-emitting diodes of 900 to 940 nm can match the response peak wavelength of silicon photoelectric devices, higher signal transmission efficiency can be obtained.

The parallel interface is also referred to as "parallel port". It is an enhanced bidirectional parallel transmission interface. The so-called "parallel port" means that 8 bits of data are transmitted simultaneously through parallel lines. The data transmission speed is greatly improved, but the length of the parallel transmission line is limited. The so-called "long line" is relative to the data transmission speed. For example, when the data transmission rate is 9600 b/s, a 20 m cable can be considered a long line. As the length of the transmission line increases, the interference will increase, errors are prone to occur, and the signal cannot be transmitted over long distances. "Isolation" and "floating" of the transmission line are better ways to solve the above problems. The use of optoelectronic isolation circuits can remove the common ground line between the two devices exchanging data, so that the two devices are electrically isolated. At the same time, in the conversion of electrical → optical → electrical signals, as long as there is a certain current at its input end, its output end can output the corresponding digital signal. Therefore, the signal transmission of the logic level becomes the state transmission of whether there is current in a fixed current loop. By appropriately increasing the current (low-resistance transmission), the electrical noise mixed in the signal is completely limited within the selected switching current amplitude, that is, the relatively weak interference signal current cannot change the presence or absence of the useful signal current, which can effectively suppress interference and improve the reliability of information transmission. And increase the transmission distance of data. Optocouplers are generally composed of light emission, light reception and signal amplification. The input electrical signal drives the light-emitting diode (LED) to emit light of a certain wavelength. It is received by the photodetector to generate photocurrent, which is further amplified and output. The conversion of electricity, light and electricity is completed, thus playing the role of input, output and isolation. Since the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectionality, so it has good electrical insulation and anti-interference ability. And because the input end of the optocoupler is a low-resistance element working in the current type, it has a strong common-mode suppression ability. Therefore, it can greatly improve the signal-to-noise ratio as a terminal isolation element in long-line information transmission. As an interface device for signal isolation in computer digital communication and real-time control, it can greatly increase the reliability of computer work.

2 Performance characteristics of optocouplers

The main advantage of the optocoupler is that it can transmit signals in one direction, the input and output ends are completely electrically isolated, it has strong anti-interference ability, long service life and high transmission efficiency. It is widely used in level conversion, signal isolation, inter-stage isolation, switching circuits, long-distance signal transmission, pulse amplification, solid-state relays (SSR), instruments, communication equipment and microcomputer interfaces. In the switching power supply, the linear optocoupler can be used to form an optocoupler feedback circuit, and the duty cycle can be changed by adjusting the control terminal current to achieve the purpose of precise voltage regulation.

The technical parameters of the optocoupler mainly include the forward voltage drop VF of the light-emitting diode, the forward current IF, the current transfer ratio CTR, the insulation resistance between the input stage and the output stage, the collector-emitter reverse breakdown voltage V(BR)CEO, and the collector-emitter saturation voltage drop VCE(sat). In addition, when transmitting digital signals, parameters such as rise time, fall time, delay time and storage time must also be considered. The current transfer ratio CTR is an important parameter of the optocoupler, usually expressed as a DC current transfer ratio. When the output voltage remains constant, it is equal to the percentage of the DC output current IC and the DC input current, IF: CTR = IC/IF × 100%.

The CTR range of an optocoupler using a phototransistor is 20% to 300% (such as 4N35), while that of PC817 is 80% to 160%, and that of Darlington optocoupler (such as 430) can reach 100% to 5000%. This shows that the latter requires a smaller input current to obtain the same output current.

3. Selection principles of optocouplers

When designing an optocoupler photoelectric isolation circuit, the model and parameters of the optocoupler must be correctly selected. The selection principles are as follows:

(1) Since the optocoupler is a unidirectional signal transmission device, and the data transmission in the circuit is bidirectional, the size of the circuit board must be certain. Combined with the actual requirements of the circuit design, it is necessary to choose a single-chip integrated multi-channel optocoupler device;

(2) The allowable range of the current transfer ratio (CTR) of the optocoupler is not less than 500%. Because when CTR < 500%, the LED in the optocoupler needs a larger operating current (> 5.0 mA) to ensure that the signal does not cause errors during long-distance transmission, which will increase the power consumption of the optocoupler;

(3) The transmission speed of the optocoupler is also one of the principles that must be followed when selecting an optocoupler. If the switching speed of the optocoupler is too slow, it will not be able to respond correctly to the input level, which will affect the normal operation of the circuit.

(4) It is recommended to use a linear optocoupler. Its characteristic is that the CTR value can be adjusted linearly within a certain range. In the design, since the input and output of the circuit are both high and low level signals, the circuit works in a nonlinear state. In linear applications, because the signal is transmitted without distortion, the appropriate static operating point should be set according to the requirements of dynamic operation so that the circuit works in a linear state.

Generally speaking, the speed of devices integrating multiple optocouplers on a single chip is relatively slow, while the devices with high speed are mostly single-channel. A large number of isolation devices need to occupy a large layout area, which greatly increases the design cost. In the design, due to the limitations of circuit board size, transmission speed, design cost and other factors, it is impossible to use a single-channel optocoupler device with a very advantageous speed. Here, TOSHIBA's TLP521-4 is selected.

4 TLP521-4 Introduction

The photoelectric isolation module TLP521-4 (GB) is a fixed-delay photoelectric coupler with excellent performance and complete base-emitter. It has the characteristics of optimal conversion speed and high temperature performance. The main characteristics of this device are: current conversion rate is 100%~500%; isolation voltage is 2500 Vrms (min); transmitting-receiving voltage is 55 V (min); leakage current is 10μA (max) (Ta=85℃); minimum conversion time is 42μs.

The typical circuit of TLP52l-4(GB) is shown in Figure 1, and the specific conversion time parameters are shown in Table 1. As can be seen from Table 1, the maximum transmission delay time of TLP521_4(GB) is 42μs. The system needs to complete the reading or writing of 8 bytes within 1 ms. The maximum transmission delay time has met the level of the circuit transmission delay time, so the transmission speed can fully meet the requirements of long-line transmission. By controlling its input end, the optocoupler can be turned on or off according to work needs. When a high level is added to the input control end, the optocoupler works normally. The input signal is coupled to the output end, and when a low level is added to the input control end, the output collector open-circuit transistor is cut off and presents a high impedance state to the outside.

5 Circuit Design

In long-distance transmission, it is precisely because of the AC impedance characteristics of the ground wire that the ground wire becomes the actual largest noise source in the circuit. The main reason for the interference caused by the ground wire is that the ground wire has impedance. When current flows through the ground wire, a voltage will be generated on the ground wire, which is the ground wire noise. Driven by this voltage, a ground wire loop current will be generated, forming a ground loop interference. Because the sending and receiving devices share a ground wire, a common impedance coupling will be formed. The use of the optocoupler TLP521-4 to electrically isolate the sending and receiving devices has a very obvious effect on reducing the AC impedance, thereby increasing the transmission current and effectively suppressing the ground wire noise; at the same time, due to the application of 74LS244N. The bus driving capability is guaranteed. Figure 2 is a schematic diagram of the optocoupler sending and receiving circuit. In Figure 2, the two optocouplers in the upper half transmit signals from left to right, and the two optocouplers in the lower half transmit signals from right to left. The left end 74LS244N exchanges data with the processor through the static memory IDT7132, and the right end exchanges data with the processor through 8255.

During debugging, a +5 V square wave with a period of 100 μs and a duty cycle of 1/2 was input, and the waveform of the input end of one optocoupler, the waveform of the output end of the receiving optocoupler after passing through a 20 m long line, and the waveform after being shaped by 74LS244 were recorded. The recording results are shown in Figure 3.

From the comparison of input and output waveforms, the circuit can significantly suppress the noise interference superimposed on the input waveform, making the output waveform smooth and stable, and improving the output signal-to-noise ratio. Although the duty cycle of the waveform has changed slightly due to the conversion time of the optocoupler, since the input and output of the circuit are both high and low level signals, it does not affect the correct transmission of the signal.

6 Conclusion

The above method is applied to the parallel port long-line transmission circuit, which can ensure the accuracy of signal transmission while maintaining the speed advantage of parallel port transmission and the unchanged system structure. In long-distance parallel port transmission, as long as the original short-distance parallel port circuit is slightly modified, high-speed isolation of the communicating parties can be guaranteed and communication can be carried out, so it has great practical value.