RS-232是一种串行通信协议，用于在计算机和其他外部设备之间进行数据传输。RS-232定义了电气特性、信号级别、机械特性和传输速率等规范，为串行通信提供了一种标准化的接口。
RS-232通常使用DB9连接器，用于传输和接收数据、控制信号以及地线连接。
但除了235脚其它基本都省略了，一个发送一个接收还有个地。
rs232是单端传输，还有485，422 差分传输，485半双工，422 全双工，原理差不多
下面是verilog代码。

tx.v

module  tx (
    input wire clk,
    input wire rst_n,
    input wire tx_begin,
    input wire[7:0] tx_pdata,
    output reg txd,
    output reg busy
);

localparam OSC = 50_000_000;
localparam BAUD = 115200;
//localparam BAUD = 57600;
//localparam BAUD = 38400;
//localparam BAUD = 19200;

localparam NUM = OSC/BAUD - 1;

reg[16:0] clk_cnt;

reg [1:0] stage;
reg [2:0] curbit;
reg [7:0] tx_data;
reg done;


always @(posedge clk or negedge rst_n  ) begin
    if (!rst_n ) begin
        busy <= 1'b0;
    end else if (tx_begin === 1'b1 && busy === 1'b0  ) begin // busy ignore
        busy <= 1'b1;
        tx_data <= tx_pdata;
    end else if (done) 
      busy <= 1'b0;
end

reg tx_clk_en;

always @(busy) begin
    if(busy)
        tx_clk_en <= 1'b1;
    else
        tx_clk_en <= 1'b0;
end

reg tx_stage_pulse;
always @(posedge clk or negedge rst_n ) begin
    if (!rst_n)
      clk_cnt <= 0;
    else if( tx_clk_en === 1'b0 || clk_cnt === NUM)
        clk_cnt <= 0;
    else 
        clk_cnt <= clk_cnt + 1;
end

always @(posedge clk) begin
    if (clk_cnt === 1)
      	tx_stage_pulse <= 1'b1;
      else
        tx_stage_pulse <= 1'b0;
end

always @(posedge clk or negedge rst_n) begin
    if(!rst_n) begin
        txd <= 1'b1;
        done <= 1'b0;
        stage <= 2'd0;
    end else if (busy) begin
        if (tx_stage_pulse) begin
            if (stage === 2'd0) begin
                txd <= 1'b0;
                done <= 1'b0;
                stage <= 2'b1;
                curbit <= 3'd0;
            end else if (stage === 2'b1) begin
                txd <= tx_data[curbit];
                if (curbit === 3'd7)
                    stage <= 2'd2;
                else
                    curbit <= curbit + 1;
            end else if (stage === 2'd2) begin
                txd <= 1'b1;
                stage <= 2'd3;
              end else if (stage === 2'd3) begin
                txd <= 1'b1;
                done <= 1'b1;
                stage <= 2'd0;
              end
        end
    end else begin
        txd <= 1'b1;
        done <= 1'b0;
        stage <= 2'd0;
    end
end

endmodule

测试
uart_test.v

module uart_test (
    input wire clk,
    input wire rst_n,
    output wire txd
);

reg tx_begin;
reg[7:0] tx_data = 0;

reg[31:0] cnt;
parameter maxcnt = 32'd1500 ;
wire busy;

always @(posedge clk or negedge rst_n) begin
    if(!rst_n)
        cnt <= 0;
    else if (cnt == maxcnt) begin
        cnt <= 0;
        if(!busy) 
        	tx_data <= (tx_data + 1) & 8'h7f ;
        tx_begin <= 1'b1;
    end else begin
        tx_begin <= 1'b0;
        cnt <= cnt+1;
    end
end

tx tx1(clk,rst_n,tx_begin,tx_data,txd,busy);

endmodule

testbench
test.v

module test;

reg rst_n=1 ;

initial begin
    $dumpfile("test.vcd");
    $dumpvars(0, test);

    #10 rst_n = !rst_n;
    #30 rst_n = !rst_n;
    #100000 $finish;
end

reg clk = 0;
always #1 clk = !clk;

wire txd;
uart_test uart1(clk,rst_n,txd);

endmodule

两个问题，一帧数据传输完成前，再次接到发送指令，直接丢弃，因为中间即使停止， 对方也会以为这是数据直到完成这一帧 ， 有可能会打乱时序 ，或用FIFO。引出一位busy忙标志位。
二停止位，有1位，1.5位，2位的，测试了下快速发送，用1位115200 57600没有乱码，速度降下去后会出现乱码，两位停止位乱码消失，稳定起见，停止位两位。