MSM7512B is a single-chip FSK modem. It conforms to the ITUT V.23 standard and can support two data transmission methods: 1200 b / s half-duplex or 1 200 b / s receive / 75 b / s full-duplex. MSM7512B is more convenient for control or one-way data communication. Such a system is often composed of a compact single-chip microcomputer and the processing control terminal is composed of a computer. Such as remote meter reading through the network, remote alarm, remote detection and remote in-system programming. But if you need remote two-way data communication through the analog access network, the problem is not simple. This leads to the problem of how to use MSM7512B for remote communication between a PC and a microcontroller.
1 Communication scheme and mode
The communication scheme should be determined by the specific application. When the PC and the single-chip computer are to communicate through the public telephone exchange network, because the MSM7512B has no additional functional circuits, it is necessary to add a dial circuit at the initiating connection and a ringing detection circuit at the answering connection. If the connection request is initiated by the computer, its communication scheme can be shown in Figure 1.
Figure 1 Communication block diagram
The 14 and 15 pins MOD1 and MOD2 of MSM7512B are the control of communication working mode. When MOD2 = 0 and MOD1 = 0, the chip is in the sending mode, and the data can only be sent out at 1 200 b / s; while MOD2 = 0, MOD1 = 1, it is in the receiving mode and can be sent at 1 200 b / s Receive data at a rate, or send data at a rate of 75 b / s. It is a pity that MSM7512B does not have a mode that can receive data at 75 b / s. Therefore, when both the PC side and the single-chip side use MSM7512B, it is impossible to send and receive data in full duplex.
However, the data transmission between the computer and the MCU is bidirectional. If there is no response confirmation, the communication process is difficult to continue. This leads to the problem of how to use two MSM7512Bs to implement bidirectional communication at the link layer with half-duplex at the physical layer. In other words, one side uses MOD2 = 0, MOD1 = 0, and the other side uses MOD2 = 0, MOD1 = 1, and these two modes need to be continuously converted during the communication process.
2 Problems that must be paid attention to in mode conversion
According to the design scheme proposed above, it is found that the simplex mode in both directions is feasible through the one-way communication test. But this does not mean that half-duplex communication must be feasible. When the modem mode is changed during the communication process after the connection is established to change the direction of transmission and reception, the received data will sometimes be correct and incorrect. Obviously, changing the mode in the communication process is a question worth discussing. After many repeated tests and analysis, it was found that the MSM7512B of the dual-machine is in the receiving state, that is, when neither of them sends data, one side can actually receive some strange data. This means that if one party has switched from the transmission mode to the reception mode and the other party has not had time to switch, the two parties will be in the receiving state at the same time (see Figure 2), and the signal on the analog channel is zero at this time, and the noise on it is mistaken by the MSM7512B It is considered that the signal is received, which results in the generation of some interference data and affects the normal communication.
Figure 2 General communication timing
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