The 8251A is a programmable universal synchronous/asynchronous receiver/transmitter. It is usually used as a serial communication interface and is widely used in microcomputers with the Intel80X86 CPU. The 8251a has two consecutive port addresses. The data input port and the data output port share the same even address, while the status port and the control port share the same odd address. Example: 8251A works in asynchronous mode. The mode selection control word is 11111011B and the work command control word is 00010001B. The initialization procedure is: ... MOV AL, 0FBH ; 8251A mode selection OUT CONTR,AL MOV AL, 11H ; 8251A operation command word OUT CONTR,AL ... Example: Hardware connection and software programming for two-machine serial communication between two microcomputers in a microcomputer system. (1) Requirements Serial communication between the two A and B computers, A machine to send, B machine to receive. It is required that the application developed on the A-machine (whose length is 2DH) be sent to B. The two parties use start-stop asynchronous mode. The data format of the communication is 8 characters, 2 stop bits, a baud rate factor of 64, no parity, and a baud rate of 4800. The CPU and the 8251A exchange data by query. The port address assignment of the 8251A is: 309H is the command/status port, and 308H is the data port. (2) Analysis Because it is a short-distance transmission, it is not necessary to set a MODEM. The two microcomputers can directly connect through the RS-232 standard interface. At the same time, the query I/O method is adopted. Therefore, in the receiving/sending program, it is only necessary to check whether the status of the send/receive ready status is set or not, and l bytes can be transmitted and received. (3) Design 1 hardware connection Based on the above analysis, both microcomputers are treated as DTEs (Data Terminal Equipment), and communications can be performed using the simplest transmission lines TxD, RxD, and GND. Using 8251A as the main chip of the interface and then configure a small number of additional circuits, such as the baud rate generator, RS-232C and TTL level conversion circuit, address decoding circuit, etc. can constitute a serial communication interface. 2 software programming It can be seen from the question that the receiving and sending programs should be written separately. Each program section includes the 8251A initialization, status query, and input/output sections. When the receiver/sender's 8251A is initialized, it is first necessary to determine the mode for selecting the control word and the work command control word. According to the requirements in the question, there are: The sender's mode selection control word is 11001111B=CFH, and the work command control word is 00111110B=37H. The receiver's mode selection control word is 11001111B=CFH, and the work command control word is 00010100B=14H. Sender's sender (omit STACK and DATA segments) CSEG SEGMENT ASSUME CS: CSEG TRA PROC FAR START: MOV DX,309H ; Control port MOV AL. 00H ; Empty operation OUT DX,AL MOV AL,40H ; Internal Reset OUT DX,AL NOP MOV AL,0CFH ;Mode word (asynchronous, 2 stop bits, 8 characters, no parity, baud rate factor 64) OUT DX, AL MOV AL, 37H ; Command word (RTS, ER, RxE, DTR, and TxEN are all set to 1) OUT DX, AL MOV CX,2DH ; Bytes transmitted MOV SI, 300H; Send Area First Address L1: MOV DX,309H ; Status port IN AL, DX; Check status bit D. (TxRDY)=1? TEST AL, 38H; Check Error JNZ ERR ;Error handling AND AL,01H JZ L1 ; Waiting to send if not ready MOV DX,308H ;Data port MOV AL,[SI] ;Send ready for sending, send 1 byte from send area OUT DX, AL INC SI ; Modify memory address DEC CX ; number of bytes minus 1 JNZ L1 ; Not sent out, continue ERR: (slightly) MOV AX, 4C00H ; Sent out, back to DOS INT 21H TRA ENDP CSEG ENDS END START Receiver receiving program (omit STACK and DATA segments) SCEG SEGMENT ASSUME CS:REC REC PROC FAR BEGIN: MOV DX,309H ; Control port MOV AL, 0AAH ; Empty operation OUT DX, AL MOV AL,50H ; Internal Reset OUT DX, AL NOP MOV AL,0CFH ;mode word OUT DX, AL MOV AL, 14H ; Command word (ER, RxE set to 1) OUT DX, AL MOV CX,2DH ; Bytes transmitted MOV DI, 400H; receiving area first address L2: MOV DX,309H ; Status port IN AL, DX; Status Bit D2 (RxRDY) = 17 TEST AL, 38H; Check Error JNZ ERR ; Turn error handling. AND AL, 02H JZ L2 ; Receive not ready, wait MOV DX,308H ;Data port IN AL, DX ; Receive ready, receive 1 byte MOV [DI], AL ; Save to receive area INC DI ; modify memory LOOP L2 ; Not completed, continue ERR: (slightly) MOV AX,4C00H ;Has been received, the program ends, exit INT 21H ; Return to DOS REC ENDP CSEG ENDS END BEGIN Moving head beam light
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8251a has several port addresses _ 8251a initialization and process
8251a internal structure diagram 8251a has several port addresses
Application Example 8251A