Sometimes I needed an external serial monitor like the Serial Monitor in the Arduino Editor, to see what is going on. So I made one.
The first picture shows an example how could be it designed.

For the ESM I used an Atmel Atmega328 (based on the Arduino-Board) because it have an internal SRAM with 2KBytes. It's necessary for the big data processing.
So you need more than 1KByte SRAM.

With the menu in the ESM you can do...
... setting the baud rate
... start and pause the reading

Step 1: Circuit

Step 2: Parts List

C1,C8,C10     100nF/50V
C2,C3    22pF/50V
C4,C5,C6             1µF
C7,C9    47µF/16V
D1,D2,D3,D4    LED-SMD OSRAM TOPLED, 9 mcd, green
Display1               EA_DOG-S102-6
IC1 ATMEGA328(DIL28S)
IC3 L7805 5V VOLTAGE CONVERTER
K1    FEMALE CONNECTOR
Q1                        16MHz
R1,R2,R3,R4     100KOhm
R5,R6,R7,R8       100 Ohm
R9                        3,3KOhm
R10                      1KOhm
S2,S3,S4,S5         JTP-1230 BUTTON
S6                        SLIDE SWITCH
9V                        Battery

Step 3: Arduino Sketch

esm.ino
The main sketch.

vars.h
Global variables are defined here.

menu.h
The menu-functions.

buttons.h
The buttons are here defined.

programs.h
The functions behind the menu.
menu()
startSerial()
settings()
mode()

myDOG.h
Functions for the DOGS-102 display.
static void setChipSelectPin(PIN No. as const int)
static void writeByte(unsigned int)
static void Init()
static void begin()
static void drawPic(byte[8][102])
static void displayClear()
static void print(Text as char*)
static void print(Text as char*, Row as int)**
static void print(Text as char*, Col as int, Row as int)**
static void print(Number long, Col as int, Row as int)**

** Col = 0 to 102 pix, Row 0 - 7

myfont.h
Font definitions and functions.

myPic.h
The start image.

mySPI.h
The SPI function.

esm.zip
Download

Step 4: Example

The left homemade Arduino board contains the following code:

/ *
sender.ino
Serial Test Program
* /

void setup () {
    Serial.begin (9600);
}

void loop () {
   for (int a = 0; a <1000; a + +) {
     Serial.print ("int a =");
     Serial.println (a);
     delay (100);
   }
}
This arduino sketch is what I created to test.
This program sends every 100 milliseconds a counter: "int a =" 0-1000.
The ESM (original Arduino board) is connected to the other Arduino via TX and RX (crossed).
Now you can press the play-button and it appears the transfer on the display.