By now you should have your Raspberry Pi and LCD running. Here we will discuss the Python code that runs the LCD display and how it works (at a top level).

On each Raspberry Pi there is a file called UMDisplay0x.py. This is the file that sends data to the LCD, it is the same on each Pi except for a setting that tells the Pi which 2 of the 8 text lines to use.

#!/usr/bin/python
#--------------------------------------
#  16x2 LCD Test Script
#
# Author : Alan Walker
# Date   : 16/05/2016
#

#--------------------------------------

# The wiring for the LCD is as follows:
# 1 : GND
# 2 : 5V
# 3 : Contrast (0-5V)*          -   Sets the LCD Contrast
# 4 : RS (Register Select)      -   GPIO pin 26 - Physical pin 37
# 5 : R/W (Read Write)          -   GROUND THIS PIN
# 6 : Enable or Strobe          -   GPIO pin 19 - Physical pin 35
# 7 : Data Bit 0             - NOT USED
# 8 : Data Bit 1             - NOT USED
# 9 : Data Bit 2             - NOT USED
# 10: Data Bit 3             - NOT USED
# 11: Data Bit 4                -   GPIO pin 13 - Physical pin 33
# 12: Data Bit 5                -   GPIO pin 06 - Physical pin 31
# 13: Data Bit 6                -   GPIO pin 05 - Physical pin 29
# 14: Data Bit 7                -   GPIO pin 11 - Physical pin 23
# 15: LCD Backlight             -   +5V**
# 16: LCD Backlight             -   GND

#import
import RPi.GPIO as GPIO
import time
import socket
import fcntl
import struct

# Define GPIO to LCD mapping
LCD_RS = 26
LCD_E  = 19
LCD_D4 = 13
LCD_D5 = 6
LCD_D6 = 5
LCD_D7 = 11


# Define some device constants
LCD_WIDTH = 40    # Maximum characters per line
LCD_CHR = True
LCD_CMD = False

# Do I need to change line 2 address for 40 char display?
LCD_LINE_1 = 0x80 # LCD RAM address for the 1st line
LCD_LINE_2 = 0xC0 # LCD RAM address for the 2nd line

# Timing constants
E_PULSE = 0.0005
E_DELAY = 0.0005

def main():
  # Main program block

  GPIO.setwarnings(False)
  GPIO.setmode(GPIO.BCM)       # Use BCM GPIO numbers
  GPIO.setup(LCD_E, GPIO.OUT)  # E
  GPIO.setup(LCD_RS, GPIO.OUT) # RS
  GPIO.setup(LCD_D4, GPIO.OUT) # DB4
  GPIO.setup(LCD_D5, GPIO.OUT) # DB5
  GPIO.setup(LCD_D6, GPIO.OUT) # DB6
  GPIO.setup(LCD_D7, GPIO.OUT) # DB7

##########################################
#             PROGRAM  START             #
##########################################


  # Initialise display
  lcd_init()

  lcd_string(" Waiting 5 Seconds for Network Start",LCD_LINE_1) # wait 5 seconds for DHCP process to finish
  time.sleep(5)


  while True:
    ## Open the file with read only permit
    f = open('/home/pi/Python/my_data.txt', "r")

    ## use readlines to read all the lines in the file
    ## The variable "lines" is a list containing all lines
    # readline 400 is read in 400 chars
    # replace removed the carriage return and new line
    # [:40] truncates to 40 chars

    line1 = f.readline(400).replace("\r\n","")[:40]
    line2 = f.readline(400).replace("\r\n","")[:40]
    line3 = f.readline(400).replace("\r\n","")[:40]
    line4 = f.readline(400).replace("\r\n","")[:40]
    line5 = f.readline(400).replace("\r\n","")[:40]
    line6 = f.readline(400).replace("\r\n","")[:40]
    line7 = f.readline(400).replace("\r\n","")[:40]
    line8 = f.readline(400).replace("\r\n","")[:40]

    ## close the file after reading the lines.
    f.close()

#    Display the current IP Address information
#    lcd_string("  lo" + " " + get_ip_address('lo'),LCD_LINE_1)
#    lcd_string("  UMD 04",LCD_LINE_1)
#    lcd_string(" eth0" + " " + get_ip_address('eth0'),LCD_LINE_2)

#    time.sleep(2) # x second delay

    # write line 1 and 2 to the LCD (Line7 and Line8 because this is UMD4)
    lcd_string(" " + line1,LCD_LINE_1)
    lcd_string(" " + line2,LCD_LINE_2)

    time.sleep(1) # x second delay


##########################################
#     CHANGE NO CODE BELOW THIS LINE     #
##########################################


def get_ip_address(ifname):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    return socket.inet_ntoa(fcntl.ioctl(
        s.fileno(),
        0x8915,  # SIOCGIFADDR
        struct.pack('256s', ifname[:15])
    )[20:24])


def lcd_init():
  # Initialise display
  lcd_byte(0x33,LCD_CMD) # 110011 Initialise
  lcd_byte(0x32,LCD_CMD) # 110010 Initialise
  lcd_byte(0x06,LCD_CMD) # 000110 Cursor move direction
  lcd_byte(0x0C,LCD_CMD) # 001100 Display On,Cursor Off, Blink Off
  lcd_byte(0x28,LCD_CMD) # 101000 Data length, number of lines, font size
  lcd_byte(0x01,LCD_CMD) # 000001 Clear display
  time.sleep(E_DELAY)

def lcd_byte(bits, mode):
  # Send byte to data pins
  # bits = data
  # mode = True  for character
  #        False for command

  GPIO.output(LCD_RS, mode) # RS

  # High bits
  GPIO.output(LCD_D4, False)
  GPIO.output(LCD_D5, False)
  GPIO.output(LCD_D6, False)
  GPIO.output(LCD_D7, False)
  if bits&0x10==0x10:
    GPIO.output(LCD_D4, True)
  if bits&0x20==0x20:
    GPIO.output(LCD_D5, True)
  if bits&0x40==0x40:
    GPIO.output(LCD_D6, True)
  if bits&0x80==0x80:
    GPIO.output(LCD_D7, True)

  # Toggle 'Enable' pin
  lcd_toggle_enable()

  # Low bits
  GPIO.output(LCD_D4, False)
  GPIO.output(LCD_D5, False)
  GPIO.output(LCD_D6, False)
  GPIO.output(LCD_D7, False)
  if bits&0x01==0x01:
    GPIO.output(LCD_D4, True)
  if bits&0x02==0x02:
    GPIO.output(LCD_D5, True)
  if bits&0x04==0x04:
    GPIO.output(LCD_D6, True)
  if bits&0x08==0x08:
    GPIO.output(LCD_D7, True)

  # Toggle 'Enable' pin
  lcd_toggle_enable()

def lcd_toggle_enable():
  # Toggle enable
  time.sleep(E_DELAY)
  GPIO.output(LCD_E, True)
  time.sleep(E_PULSE)
  GPIO.output(LCD_E, False)
  time.sleep(E_DELAY)

def lcd_string(message,line):
  # Send string to display

  message = message.ljust(LCD_WIDTH," ")

  lcd_byte(line, LCD_CMD)

  for i in range(LCD_WIDTH):
    lcd_byte(ord(message[i]),LCD_CHR)

if __name__ == '__main__':

  try:
    main()
  except KeyboardInterrupt:
    pass
  finally:
    #lcd_byte(0x01, LCD_CMD)
    #lcd_string("Goodbye!",LCD_LINE_1)
    GPIO.cleanup()