In order to avoid interference with the machine during transmission, we have adopted half-duplex communication. The so-called half-duplex communication refers to that the master and slave can perform two-way communication, but not simultaneously. At the same time, either the master transmits and the slave receives; or the slave transmits and the master receives. In fact, although the center frequency of the receiving carrier of the general infrared receiving module is 38KHz, its receiving frequency is actually very wide, and it can be correctly received and demodulated from 30KHz to 60KHz. Therefore, it is difficult for the master and slave to use different carrier frequencies to achieve full-duplex communication unless an infrared receiving circuit with better frequency selection characteristics is used. In the infrared transmission process, negative logic is used, that is, infrared means logic 0, and no infrared means logic 1. The infrared receiving module is usually at a high level, and it becomes a low level when the infrared signal is received.
How to increase the transmission distance? There are two methods: one method is to use a high-sensitivity infrared receiving circuit, but its cost is relatively high, and it is relatively susceptible to interference. Another method is to increase the transmit power (referring to the peak power, it is not necessary to increase the average power). This can be achieved by reducing the resistance R connected to the transmitter, which can usually be reduced to 10Ω. However, it should be noted that in this case, in order to reduce power consumption and interference with other infrared devices, the duty cycle of the infrared carrier needs to be reduced. Generally choose a duty ratio of about 30%, that is, the high time of DATCLK should be about 18uS, and the low time should be about 8uS (38KHz carrier frequency)
two. PC infrared receiving principle
1. Brief introduction of RS232 standard In PC, infrared communication port is generally integrated, but because of its complicated communication protocol, it is inconvenient to interface with MCU. For the sake of simplicity, we use an extension of an infrared receiving circuit on the COM port (RS232 port) of the PC to achieve. Here, first a brief introduction to the RS232 standard.
RS232 is a serial communication recommendation standard formulated by EIA (Electronic Industry Association), used for the connection between DTE and DCE (typical DTE is PC, typical DCE is modem), the effective transmission distance is 15 meters, if 4 ~ 20mA is used The current loop can reach a communication distance of more than 1 km. In RS232, a total of 25 signal lines (including data lines, control lines, status lines, etc.) are specified, but in actual applications, not all signal lines are necessarily used. In the early PCs, there was usually a 25-pin serial port and a 9-pin serial port; in the PCs produced today, both serial ports use a 9-pin DB9 socket. The signal line definition of the 9-pin serial port is shown in Table 1:
2. Infrared receiving expansion circuit Here we think that DTE is PC, then we can see from Table 1, there are three PC output pins: TXD, DRT and RTS. TXD is a serial data transmission port with a baud rate from 110bps to 115200bps (hardware support required). DTE uses DTR to tell DCE that it is ready to receive data, and uses RTS to request DCE to send data. These two signals are mainly used for hardware flow control. In the infrared receiving extension circuit, flow control is not required, but DTR and RTS are used to provide power for the receiving circuit. We know that the standard RS232 level is negative logic: + 3V ~ + 15V means logic 0; -3V ~ -15V means logic 1. In this way, as long as DTR and RTS are enabled, these two pins can output a level of about + 15V, thereby supplying power to the infrared receiving circuit. According to this design idea, the infrared receiver expansion circuit is shown in Figure 1:
In the above figure, D1, D2 and D4 play an isolation role, R1 is mainly used to protect the PC serial port, C1 and C2 are used for filtering, and U1 is a three-terminal regulator that provides + 5V working power for the infrared receiving module. U2 is a three-terminal integrated infrared receiving module. The general receiving center frequency is 38KHz, which can complete the entire process from infrared receiving, filtering to demodulation. The LED acts as an indicator light, and the LED will flash when an infrared signal is received. The key point of this circuit is the connection of the three pins DTR, RTS and TXD: DTR and RTS are power supply pins, which can provide about 15V of power, provided that they are enabled in the PC monitoring program. The TXD pin provides negative power for reception. The TXD pin is a data sending pin, why can it provide negative power? This is the ingenuity of this circuit. We know that in the RS232 protocol standard, when no data is sent, the TXD pin outputs a low level, that is, about -15V. In the infrared receiving extension circuit, it only has infrared receiving function, but no transmitting function, so it can use the TXD pin to provide low level for receiving. When no infrared signal is received, pin 1 of the infrared receiver module outputs a high level, the transistor Q1 is turned off, and TXD pulls the RXD pin to a low level through D4; when the infrared signal is received, the infrared receiver module outputs a low level , Transistor Q1 turns on, at this time the RXD pin becomes high level.
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Designed for memory backup and standby applications, Antenk's compact coin cell battery retainers permit quick and easy coin cell replacement and installation. By eliminating soldered-in cells, computer, video, telecommunication, and similar PCB based product users now have a reliable, no tools required method for changing batteries.
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Available in THM or SMT configurations
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