プチロボ用シリアル通信のC言語コードのサンプルを見直しましたので、ご一報まで。

C:\> wsl --set-version Ubuntu-18.04 1

/***********************************************************************************************************
 * WR-XX Dummy for Linux ver. 0.2.2
 * Author : Lian Ivvakanni (01-03-2021)
 * License: Dual BSD/GPL (with absolutely no warranty.)
 * Purpose: send return message to Wonder Roid Motion Maker V3.1.1
***********************************************************************************************************/
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

/* change this definition for the correct port */
#define MODEM_DEVICE "/dev/ttyS3"
#define BAUD_RATE   B38400
/* command item number  */
#define M 8
/* buffer size  */
#define N 32

#define HEADER 0xFFFFFFFD
#define FOOTER 0xFFFFFFFE  

int main(int argc, char *argv[])
{
    int  fd, i, j, k, len;
    struct termios tio;
    char s[N], sprn[N], buf[N], ret[M][N];
/*  Command 5: A/D status dummy data  */
    char dum_5[] = {0xFD,0x0F,0x15,0x46,0x46,0x46,0x46,0x46,\
                    0x46,0x46,0x46,0x46,0x46,0x00,0xFE};
/*  set return data  */
    for (i = 0; i <= 7; i++)
        ret[i][2] = i + 0x10;
/*  clear structure for port settings  */
    memset(&tio, 0, sizeof(tio));
/* 
    BAUDRATE: 38400 bps
    CS8     : 8bit,no parity,1 stopbit (8n1)
    CLOCAL  : local connection, no modem contol
    CREAD   : enable receiving characters
*/
    tio.c_cflag = BAUD_RATE | CS8 | CLOCAL | CREAD;
/*  Open modem device for reading and writing */
    fd = open(MODEM_DEVICE, O_RDWR);
    if (fd < 0) {perror(MODEM_DEVICE); exit(-1);}
    printf("Device Opened...\n");
/*  clean the modem line and activate the settings for the port  */
    tcflush(fd, TCIFLUSH);
    ioctl(fd, TCSETS, &tio);

    while(1){
    /*  clear buffer memory  */
        memset(buf, '\0', N);
    /*  open device to read  */
        len = read(fd,buf, sizeof(buf));
        if(len < 0) printf("Device Read Error...\n");
    /*  find header  */
        if(buf[0] != HEADER) 
            continue;
    /*  check command data size  */ 
        if(buf[buf[1] - 1] != FOOTER) 
            continue;
    /*  optional: set buffer to string  */ 
        for (i = 0; i < buf[1]; i++){
            sprintf(s, "%02X ", buf[i]);
            strcat(sprn,s);
        }
        printf("Rec: %s: %d\r\n", sprn, i);
        memset(sprn, '\0', sizeof(sprn));
    /*  find command and send its return, if any */
        if(buf[2] == 0x00) 
            write(fd, ret[0], 4); 
        if(buf[2] == 0x02) 
            write(fd, ret[2], 4); 
        if(buf[2] == 0x03)
            write(fd, ret[3], 4); 
        if(buf[2] == 0x05) 
            write(fd, dum_5, 15);
        len = 0;
    }
    close(fd);
    return 0;
}