#include "mbed.h"
#include "iSerial.h"

char s[] = "12345678901234567890123456789012345678901234567890";

int main() {

    iSerial pc(USBTX,USBRX);
    char* c = s;

    while(1) {
        pc.puts(s);
        
        while(pc.readable()){
            int ret = pc.getc();
            if(ret)
                *c++ = ret;
            if(c >= s+sizeof(s)-1)
                c = s;
        }
        wait(1);
    }
}

Comparing speed of ordinary Serial functions

#include "mbed.h"
#include "iSerial.h"

char s[] = "12345678901234567890123456789012345678901234567890";    // 50 bytes

int main() {

    Timer t;

    Serial pc(USBTX,USBRX);
    pc.baud(921600);
    
    t.reset();
    t.start();
    for(int i=0; i<30; i++) {
        pc.printf(s);
        wait(0.001);
    }
    t.stop();
    
    int t1 = t.read_us();

    pc.puts("\n\n\n");


    iSerial ipc(USBTX,USBRX,NULL,1000,100); // try NOT to set the buffer over flowed
//    iSerial ipc(USBTX,USBRX);   // set as default = 100, 100
    ipc.baud(921600);

    t.reset();
    t.start();
    for(int i=0; i<30; i++) {
        ipc.printf(s);
        wait(0.001);
    }
    t.stop();
    
    int t2 = t.read_us();

    ipc.printf("\n\n\nThe time %d, %d [us] was taken.\n", t1, t2);
    ipc.puts("End.");
    ipc.flush();
}