实验内容

通过 E 命令修改内存 3000H 的内容，发送到串口，然后自接受保存到 4000H 起始的内存单元中。

最后通过系统命令查看相应存储空间的数据，验证传输结果。

实验接线

通过 8254 控制波特率，将 TXD 和 RXD 相连以自收发。

代码

DATA8251 EQU 0600H
CON8251 EQU 0602H

C028254 EQU 06C4H
CON8254 EQU 06C6H

SSTACK SEGMENT STACK
  DW 64 DUP(?)
SSTACK ENDS

CODE SEGMENT
ASSUME CS:CODE

START PROC
  ;; INIT 8254
  MOV DX, CON8254
  MOV AL, 10110110B ;; MODE 3, SQUARE
  OUT DX, AL
  
  ;; CLK2 CONNECTED TO 1.8432MHz
  ;; OUT2 CONNECTED TO 8251-TRCLK
  ;; 1.8432E6/0X000C/16 = 9600BPS
  MOV DX, C028254
  MOV AL, 0CH
  OUT DX, AL
  MOV AL, 00H
  OUT DX, AL
  
  ;; INIT 8251
  CALL INIT
  MOV DX, CON8251
  MOV AL, 01111110B ;; ASYNC METHOD CONTROL WORD
                    ;; 01: ONE BYTE ESD/S1
                    ;; 11: EVEN JUDGE
                    ;; 11: 8 BYTES
                    ;; 10: 16
  OUT DX, AL
  CALL DELAY
  MOV AL, 00110100B ;; ASYNC COMMAND WORD
                    ;; 0: DISALLOW SEARCH
                    ;; 0: NO RESET
                    ;; 1: REQUEST SEND
                    ;; 1: RESET ERROR FLAG
                    ;; 0: NORMAL WORK
                    ;; 1: ALLOW RECEIVE
                    ;; 0: DATA CONSOLE NOT PREPARED
                    ;; 0: DISALLOW SEND
  OUT DX, AL
  CALL DELAY
  
  MOV DI, 4000H
  MOV SI, 3000H
  MOV CX, 10

LO:
  MOV AL, 00110111B ;; ASYNC COMMAND WORD
                    ;; DIFFERENCE: ALLOW SEND
  MOV DX, CON8251
  OUT DX, AL        ;; PREPARE 8251
  MOV DX, DATA8251
  MOV AL, [SI]
  OUT DX, AL        ;; SEND [SI] BYTE

  MOV DX, CON8251
LO_SEND:
  IN AL, DX ;; WAIT FOR SEND BUFFER TO BE EMPTY
  AND AL, 1
  JZ LO_SEND

LO_RECV:
  IN AL, DX ;; WAIT FOR RECV BUFFER TO HAVE DATA
  AND AL, 02H
  JZ LO_RECV

  MOV DX, DATA8251
  IN AL, DX     ;; RECEIVE FROM 8251
  MOV [DI], AL  ;; SAVE TO [DI]
  INC DI
  INC SI
  LOOP LO

FIN:
  JMP FIN

  MOV AX, 4C00H
  INT 21H
START ENDP

INIT PROC
  PUSH AX
  PUSH DX
  MOV AL, 00H
  MOV DX, CON8251
  OUT DX, AL
  CALL DELAY
  OUT DX, AL
  CALL DELAY
  OUT DX, AL
  CALL DELAY
  MOV AL, 40H
  OUT DX, AL
  CALL DELAY
  POP DX
  POP AX
  RET
INIT ENDP

DELAY PROC
  PUSH CX
  MOV CX, 0FFFFH
  LOOP $
  POP CX
  RET
DELAY ENDP

CODE ENDS
END START

实验过程

设置内存

4000H

3000H

选做实验

说明

将 a-z 的 ASCII 码写入 3000H，然后通过 9600 的波特率传输到 4000H。

当出现错误时停止传输，并将状态字显示在数据灯 D7-D0 上。

接线

接线有点混沌，反正能用就行（

由于数据线低位不够用了，所以我把 8255 接到了数据线高位（D8-D15），对应的 OUT 也就需要输出 AX 而非 AL 了。

总之就是下面这个样子：

代码

DATA8251 EQU 0600H
CON8251 EQU 0602H

C028254 EQU 06C4H
CON8254 EQU 06C6H

B8255 EQU 0642H
CON8255 EQU 0646H

SSTACK SEGMENT STACK
  DW 64 DUP(?)
SSTACK ENDS

CODE SEGMENT
ASSUME CS:CODE

START PROC  
  ;; INIT 8254
  MOV DX, CON8254
  MOV AL, 10110110B
  OUT DX, AL
  
  MOV DX, C028254
  MOV AL, 0CH
  OUT DX, AL
  MOV AL, 00H
  OUT DX, AL
  
  ;; INIT 8251
  CALL INIT
  MOV DX, CON8251
  MOV AL, 01111110B
  OUT DX, AL
  CALL DELAY
  MOV AL, 00110100B
  OUT DX, AL
  CALL DELAY
  
  ;; INIT 8255
  MOV AL, 10010000B
  MOV DX, CON8255
  OUT DX, AL
  
  ;; INIT [3000]
  MOV SI, 3000H
  MOV AL, 61H   ;; a
  MOV CX, 26
INIT_CHAR:
  MOV [SI], AL  ;; SAVE TO [SI]
  INC AL
  INC SI
  LOOP INIT_CHAR

  MOV SI, 3000H
  MOV DI, 4000H
  MOV CX, 26

LO:
  MOV AL, 00110111B ;; ASYNC COMMAND WORD
                    ;; DIFFERENCE: ALLOW SEND
  MOV DX, CON8251
  OUT DX, AL        ;; PREPARE 8251
  MOV DX, DATA8251
  MOV AL, [SI]
  OUT DX, AL        ;; SEND [SI] BYTE

  MOV DX, CON8251
LO_SEND:
  IN AL, DX ;; WAIT FOR SEND BUFFER TO BE EMPTY
  AND AL, 1
  JZ LO_SEND

LO_RECV:
  IN AL, DX ;; WAIT FOR RECV BUFFER TO HAVE DATA
  TEST AL, 02H
  JZ LO_RECV
  
  TEST AL, 00111000B
  JNZ ERR_FIN

  MOV DX, DATA8251
  IN AL, DX     ;; RECEIVE FROM 8251
  MOV [DI], AL  ;; SAVE TO [DI]
  INC DI
  INC SI
  LOOP LO

FIN:
  JMP FIN

ERR_FIN:
  ;; OUTPUT TO 8255
  MOV DX, B8255
  MOV AH, AL
  OUT DX, AX

  MOV AX, 4C00H
  INT 21H
START ENDP

INIT PROC
  PUSH AX
  PUSH DX
  MOV AL, 00H
  MOV DX, CON8251
  OUT DX, AL
  CALL DELAY
  OUT DX, AL
  CALL DELAY
  OUT DX, AL
  CALL DELAY
  MOV AL, 40H
  OUT DX, AL
  CALL DELAY
  POP DX
  POP AX
  RET
INIT ENDP

DELAY PROC
  PUSH CX
  MOV CX, 0FFFFH
  LOOP $
  POP CX
  RET
DELAY ENDP

CODE ENDS
END START

结果

3000H

4000H

演示：数据灯

由于传输过程中没有出现问题，因此没法检测 8255 的部分是否正确。因此这里强行测试了一下（

修改上述代码的 79 行，把 JNZ 修改为 JZ。这样就能输出正确情况下的状态字了。实际的 LED 输出如下图所示：

通过 LED 我们可以读出当前的状态字：10000111。