The front-end machine is based on an 8031 single-chip microcontroller system equipped with an RS-422 interface, and it is located close to the control site. The communication controller acts as an interface board that contains an 8031 microcontroller inserted into the IBM-PC/XT expansion slot. This setup provides an intelligent communication channel between the host and the front-end device. Users only need to send simple commands from the host to the controller without needing to understand the underlying communication details. As a result, the front-end machine can execute the corresponding actions. Each time the controller sends or receives data, the amount is variable but does not exceed 255 bytes. Error detection and error handling are implemented for every communication to ensure the accuracy of data transmission. The IBM-PC/XT serves as the command initiator and the system controller.
Figure 1-27 shows a block diagram of the front-end machine, which is a 51-series microcontroller system with an RS-422 serial interface. The number of input/output and A/D conversion channels is configured based on the specific needs of the application control system, such as an automatic car testing line, and these configurations are independent of the communication process. Figure 1-28 illustrates the serial interface schematic. The interface between each front-end machine and the communication controller uses differential driving and receiving through MC3486 and MC3487. The serial bus consists of a pair of twisted wires labeled DATA+ and DATA-. At the end of the twisted pair, there is a pull-up resistor that ensures the line remains in a logic 1 state when idle. When a logic 0 (TxD=0) is transmitted, the line switches to a logic 0 state. This differential drive and receive bus meets the electrical specifications of the RS-422 standard. Compared to RS-232, this type of bus offers better noise immunity, faster data transmission rates, and longer transmission distances.
Figure 1-29 presents the block diagram of the communication controller. The 8031 communicates with both the front-end machines and the host through its serial and parallel interfaces. The parallel interface schematic is shown in Figure 1-30. Data exchange between the controller and the IBM-PC/XT is done using a polling method. The 8255A operates in Port A mode 2, which is a bidirectional bus I/O mode. When the controller sends data to the IBM-PC/XT, the WR signal becomes active, allowing data to be written to Port A, which sets OBFA (PC7) to 1 (initially OBFA = 0). The IBM-PC/XT then checks the status of 8212III and detects that OBFA = 0, after which it reads the data from Port A. At the same time, the 8212I INT signal resets OBFA back to 0. When the IBM-PC/XT sends data to the controller, it writes the data to 8212II using an output command. Simultaneously, the 8212II INT signal becomes active, generating a strobe signal on PC4 (STBA), which sets PC5 (IBFA) to 1 (initially IBFA = 0). The 8031 then checks the C port of the 8255A and detects that IBFA = 1, at which point it reads the data from Port A. The parallel port address is partially decoded, with 8212I and 8212II located at address 02EOH, and 8212III at 02EIH.
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