Abstract Descriptions of the physical layer bus, on the basis of technical parameters given P89C51RD2 MCU and P300P111 sweep consisting of power line carrier communication module design, with emphasis on the power line communication module interface circuit design of filter circuits that are involved in , amplifier circuit, coupled circuits and the protective measures.
Intelligent home appliances required by the network (bus) to achieve interconnection, interoperability, bus protocol is its essence. Currently, the dominant international standard home network are the United States X10 [1], consumption of the bus (CEBus) [2], Japan's family bus (HOME BUS) [3], the European Installation Bus (EIB) [4 ].
Consumer bus uses five types of media (power line, wireless, infrared, twisted pair and coaxial cable), of which the most widely used power lines. Consumption of the bus by IBM, Hownywell, Microsoft, Intellon, Lucent, Philips, Siements the support of other large companies in 1992 to become the standard Electronic Industry Association of America (EIA600, EIA721). In 1997, EIA600 became the ANSI standard; June 2000, Microsoft and the Commission jointly announced their support for CEBus simple CEBus control protocol SCP. SCP is the future of micro-subset of the UPNP protocol.
1 CEBus Power Line Physical Layer
In view of the family in the special power line carrier communications, CEBus uses inexpensive, simple linear FM (chirp) spread spectrum modulation techniques. Abandon the traditional application of power line carrier is usually a direct sequence spread spectrum, frequency spectrum, time-hopping spread-spectrum devices such as complex, expensive spread spectrum modulation techniques.
Figure 2 General schematic of the communication module Click to enlarge
Consumer Bus physical layer has four codes, namely 0, 1, EOF and EOP. Are the frequency sweep signal, sinusoidal signal carrier, after 19 cycles from 203kHz linearly into 400kHz, then after a period into a 100kHz, and then in 5 cycles into 203kHz, with the whole process when 100µs, which is 1 UST (Unit symble time, the bus used in the consumer to measure the number of UST time). The waveform shown in Figure 1.
chirps to be swept carrier coupled to the power line through the amplification, the amplified increase should be moderate. Rate is too low, to the receiving circuit difficult; magnitude too large, and the equipment of the power line interference. Table 1 CEBus requirements [5] shows.
Table 1 under different conditions, the value of carrier amplitude
Device operating voltage range of the minimum amplitude of the maximum amplitude of the load
~ 120V2.5Vpp7Vpp10O ~ 2kO
~ 240V5Vpp14Vpp39O ~ 8.2kO
Table 2 under different conditions, the input impedance of the device
Device input impedance voltage equipment (in the frequency of 20kHz ~ 50000kHz) carrier amplitude
~ 120V 150O6Vpp
~ 240V 300O12Vpp
Electrical equipment and also provides the signal impedance. If the impedance is very small, and thus would not send the signal absorption countries. Provided in Table 2 [5] shows.
Chirp technology to achieve low power density of broadband transmission, thus greatly improving the noise immunity and transmission distance. At the same time, chirps has a strong correlation between self and self-synchronization. That all relevant decisions from the connected devices on the network can identify any device from the Internet sent this particular waveform.
2 Communication Module
According to P89C51RD2 and P300 chip Manual [6] [7], the design principle of universal communication module shown in Figure 2. P89C51RD2 and SPI interface to communicate between the use of P300, using a I2C bus and Serial EEPROM communications. Thus, the interrupt port, serial port, and have enough I O ports can be used in actual equipment design.
3 power line communication module interface circuit design
P300 amplitude output signal from the small drive capacity is weak but also higher harmonics, it must be filtered and amplified, then the signal can be modulated by coupling the circuit to the power line. Coupling circuit to isolate the high and low pressure to prevent the breakdown of communications high-voltage circuit. On the other hand, come from the power line carrier signal and also receives from the P300, and a large power line interference is also uncertain, so a band-pass filter, through the signal between 100kHz ~ 400kHz, and then sent to the receiving P300 end. Block diagram of the circuit shown in Figure 3.
Which left three lines from P300, TS is a digital signal, control the transceiver conversion. P300 is actually similar to half-duplex transceiver, because when it sends bad state when, in fact, no output signal. Therefore, this time it is in receiving state, if the receiver to the Superior to the competition has occurred.
3.1 filter circuit
Input filter circuit shown in Figure 4.
This filter has 6 bands have a good high frequency interference suppression, Figure 5 is its frequency response curve. In the high frequency attenuation at 400kHz 3dB. Higher than 400kHz, the average attenuation of 3dB, 400kHz above the average attenuation 128dBdec, you can effectively filter unwanted signal.
P300 output signal contains a wealth of high harmonics, in order to reduce interference on the power grid, the first through the band pass filter further amplification. Filter is also used passive circuit, similar to the above principle, and is not to repeat.
3.2 Amplifier
Filtered output signal of P300 after its great resistance, there is no drive capability, and the bus voltage range does not meet the consumption requirements, must be amplified to drive after the power line. Not only have a powerful amplifier output capability, should also have prohibited the export function, so as to enable other nodes to receive P300 signal.
The performance of grid uncertainty, sometimes capacitive load, sometimes emotional load. This gives the final stage circuit is very difficult for the feedback. Because when the load impedance changes, the output signal phase will change, eventually there may be negative feedback into positive feedback, causing oscillation.
Figure 6 Power Line Amplifier Click to enlarge
A power line carrier amplifier design shown in Figure 6, dotted line to the left of the diagram, the right is the realization of the schematic. Can see that this circuit has two inputs and one output. Input signal from the P300's power line carrier, output enable control amplifier operation. Left part of Figure 6, T1 and T2 connected as complementary OTL output bias voltage from the resistance of their R1, R2 of the divider. The signal from the P300 amplifier U1 through amplification to achieve the desired range, then capacitor coupled to the base of T1 and T2. If the switch S1 and S2 closed, the T1 and T2 normal output Telecom, P300 can send data; are disconnected if the S1 and S2, then the T1 and T2 of the base are in a floating state, the output has become a suspended state, so as not to will absorb the signal coming from the power line, P300 can receive signals.
The right side in Figure 6, switch S1 and S2 have been replaced by T7 and T8, T1 and T2 were replaced by composite pipe, in which the resistor R11 is used to eliminate transistor leakage current. Composite tube is to increase the magnification, which can minimize the inter-stage coupling, even if the output signal of the distortion, it will not affect the oscillations occur before the class. Actual proof of this is very feasible. Its capacitive load, inductive load, and purely resistive load has a more stable output, the output impedance of less than 2O.
Figure 7 P300 with the power line coupling circuit Click to enlarge
Coupled circuits and protection measures 3.3
Figure 7 receives power line J1, R1 is a varistor, which can short-circuit the peak pulse transformer T1 high-pressure and low pressure to achieve the isolation. Because of relatively high carrier frequency (100kHz ~ 400kHz), the size of the grid frequency far, so smooth that the carrier signal, and to cut off high pressure. Low-frequency high-voltage blocking capacitor C1, to prevent transformer saturation; resistor R2 value is relatively large, is to discharge the capacitor when offline, to prevent the two ends of the plug in a high-pressure equipment. Z1 is a transient suppression diode (Transient Voltage Suppressor, also known as TVS), which can effectively avoid the breakdown after the circuit is high. L1, D1, D2 is to prevent high-voltage amplifier circuit designed to breakdown. Power line access or disconnect the device, are likely to cause spikes, and lead to permanent damage to the transceiver circuit. Therefore, high-pressure protection is essential.
In addition to the power line will produce damage to the device outside the high-voltage pulse, when the device has just connected to power (see Figure 7), if the voltage power line just at the maximum, at which point the voltage on the capacitor is 0, there will be 300V (220V RMS , max 311V) plus high pressure directly across the transformer, causing a large current, which produces spikes in the secondary. The large pulse of current, up to tens of amps to hundreds of amps, with a general regulator there is no way to eliminate this pulse. I once tried to absorb the pulse with varistors, the varistor response is relatively slow, in the event of a microsecond pulse still has tens of volts within the power, enough to burn amplifier. Practice shows that this just access circuit transient pulse generated considerable destructive power. Fortunately, it's current, while significant, but the energy is not so big. I used the transient-suppression diodes 1.5KE6.8CA response time is 5ns, can absorb 200A current, transient power up to 1500W. Can simply see it as a strong ability to absorb the current regulator diode, but it's dynamic resistance is relatively large, so it needs both D1 and D2 Schottky diode clamp further resistance around the power supply voltage, The action of blocking inductor L1 particularly narrow high voltage pulse. After these protective measures, the circuit does not appear after any failure