Multi-machine call system design
Traditional multi-machine communication systems generally require four lines to complete:
1. Power line; 2. Ground line; 3. Sending signal line; 4. Receiving signal line. However, for a system where the distance between the host and the extension is long and the number of extensions is large, the four-wire system requires a large investment and is difficult to install. Based on this problem, this paper combines a transmission call system that has both analog signals (voice) and digital signals developed for a hospital, and proposes using a single bus to implement multi-machine communication, and gives a complete technical solution.
1 Bus design scheme for single-bus multi-machine communication system
The main function of the multi-machine call system designed by this design is: the extension calls the host, uses a single-chip microcomputer to send the digital call information to the host, after the host responds, the extension number of the call is displayed, and the analog channel of the host and the extension is opened to perform the host and extension. Intercom; the host calls an extension, the host outputs the extension number, directly opens the analog channel of the corresponding extension, and performs the intercom of the extension and the host; the group calls, mainly sends the digital information of all the extensions one by one, and all the extensions are opened within several seconds. The analog channel, the host sends voice signals to each extension. The core issues for implementing such multi-machine communication using a single-bus system are how power, ground, and signal lines are distributed, how signals are transmitted in both directions, and how to resolve mutual interference between them. The author shared a signal (including digital signal and analog signal) with the power supply, and the other line was a ground-single-bus system. In the circuit structure and technology, the power isolation circuit is used to minimize the influence of the power supply filtering on the signal; and the analog signal processing is completed by the elimination of the side tone circuit. Host circuit block diagram shown in Figure 1.
When the host needs to talk with a certain machine, the operator picks up the microphone and dials through the keypad of a common telephone. The display shows the called extension number. At the same time, the digital signal is sent to the bus. The extension receives the digital signal and confirms the host request. While talking, switch on the analog channel. The host's voice signal is boosted by amplifiers, single-bus to extension amplifiers. The extension's voice signal is amplified by the power amplifier, eliminates the side-tone circuit, and the single-bus to host amplifier pushes the speaker or handset. After the call is completed, the host hangs up. At the same time, the host computer sends a stop display signal and an extension hang-up signal, the display goes out, and after the extension receives the hang-up signal, the analog channel of the extension is suspended to complete a call process. .
When the mainframe needs to be connected with the extensions at the same time, for example when the hospital needs to notify all the beds of something, it is unbelievable if the extensions are dialed one by one with the keyboard. This design takes into account this issue. As long as the operator presses the group call key, the extensions can be connected within a few seconds to implement the group call.
When the extension needs to talk with the host, press the call button (the extension only presses this button), and the single-chip microcomputer sends a digital signal and sends it to the single-chip microcomputer of the host via a single bus. After the host microcomputer receives the signal, it displays the extension number and judges whether the telephone hangs up. If it is in the hang-up state, the loop judges until the phone hangs. After picking up the microphone, the host immediately sends a signal to the corresponding extension to allow the call. The extension connects to the analog channel to make a call. After the call is completed, the microphone hangs up. The host sends a hang-up signal to the extension. The extension suspends the ground of the analog channel. In this way, it saves energy without affecting the normal work of some of the crews.
The priority of the communication must be given priority in the call system for the hospital's critical illness number and the critical department of the organization. In this design, after receiving a large number of call signals, the host first determines the priority, turns on the highest priority extension call, and then gradually communicates with the low-priority extension.
2 power isolationIn a single-bus multi-machine communication system, digital signals, analog signals, and power supplies share a single line. If no measures are taken, the alternating digital signals and analog signals will be absorbed by the filter capacitors of the power supply. In this design, when the power isolation circuit is not added, the alternating 5V signal is only 10mV after passing the transmission line to the extension. Using the isolation circuit shown in Figure 2, the signal is transmitted to the extension at more than 250mV, and the attenuation of the signal is significantly improved. The isolated circuit of diode, triode, voltage regulator, inductance and resistance in the figure is used for the host, and the extension only needs one inductor and one diode.
A single bus must send both voice signals and voice signals, which is bound to form a circulation. In order to solve the problem of hearing one's own voice (called a side tone) during a speech, a countermeasure was taken. Generally speaking, there is a method to eliminate sidetones: one is a bridge-type side-elimination circuit; the other is a side-tone cancellation circuit, and the latter is better. Taking into account the cost adjustment factors of a host and multiple extensions, the host uses the phase cancellation to eliminate the side tone, and the extension uses the bridge to eliminate the side tone.
The phase cancellation elimination side tone circuit is shown in Fig. 3. This circuit consists of three operational amplifiers and corresponding resistors, capacitors and potentiometers. The extension signal from the bus passes through RW1 and F3 to the power amplifier. The microphone signal of the host passes through the power amplifier 1, the bus, the non-inverting input of RW1 to F3, and the other passes the inverting input of F1, F2, RW2 to F3. Adjusting the appropriate K, C, RW1, RW2 values ​​allows the host's microphone signal to approach zero volts at the F3 output, thereby eliminating sidetones.
The bridge-type elimination side sound circuit is shown in Figure 4. The entire circuit consists of a transistor (voice transmission amplification) and R and C components. In the figure, ZL is the equivalent impedance of the external line. When the extension sends a phone signal, the emitter of the transistor outputs the single-frequency signal current, and the national road is connected to the emitter of the triode via the external line ZL and the resistor R3 to form a current loop; the other country passes R1, C1, R2 and C2. Form a current loop. If the bridge is balanced,
ZL(R2-j1/ωC2)=R3(R1-j1/ωC1) then Vc0=0
Since the input signal of the rear power amplifier is zero, the speaker is not sounded and the side sound is eliminated. The analog signal from the host through the bus is amplified by R1, C1, and RC to the voltage amplifier for power amplification.
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