The idea is to have messages from 2 serial ports being displayed and scrolled up. I would call it semi-duplex, as ideally one side sends a command/line and the the other side. If both sides send characters at the same time, they get mixed with each other in the same line. Different colours for the 2 sides are used, so it is visible if that happens.

HW Setup:
Arduino Mega with P42 Video4 VGA Shield
Standard serial port setup as SERIAL 115200 8N1
2nd serial port setup as SERIAL1 38400 8N1 (this is also a hardware serial port on the Mega on pins 18:TX 19:RX). The baudrate is set to 38400 to be operational with the Arduino SoftwareSerial library as well. For the 2nd serial port a TTl-UART adapter is required with connections to RX, TX and GND.

SW Setup:
!!! The Arduino Serial Monitor is not suitable for this setup. A proper terminal software is required, I use two instances of TeraTerm, configured with the correct serial settings, local echo and transmit CR+LF.

The display shows the 2 different sources in light and dark colour.

Description:
The global variables ScreenX and ScreenY as well as the function printlineXY() are the heart of this demo.

ScreenX, ScreenY point to the coordinate of the screen where the next character will be printed. Function: byte printlineXY ( byte channel, char* Text, byte colour, bool ln )
  Paramters:
  byte channel: is the video channel used for display; can be one of CH0, CH1, CH2, CH3.
  char* Text: pointer to a string, null terminated. To print a single character, the character + a NULL byte need to be content of the string. Maximum string length is the remainder of the current line + a full additional line as only one linebreak is currently implemented.
  byte colour: a 8-bit colour value picked from the colour table in the user guide.
  bool ln: of TRUE a CR+LF is performed to start a new line after the string got printed.    byte return value: currently always 0.

 At the bottom of the screen, if the last line is full or LF = TRUE, a line scrolling is implemented to make room for the next line of text.  Implemented control codes:
 0x08 : Backspace, the last character gets deleted to free the area for the next character to be printed, so technically it is a 'delete char'
 0x0A : LF Linefeed, cursor go to next line, but same horizontal position  0x0D : CR Carriage return, cursor goes to the beginning of the current line

SW repository
https://github.com/wolfgangfriedrich/P42-VGA/tree/master/Code/P42VGA_V4Text