The VP-EC-RDU Remote Display is the perfect addition to any controls system or project that uses the MODBUS RTU protocol and RS485. The easy to read 4 line x 20 character display is simply written to by writing the applicable holding registers, along with the status LEDs and Audible. The four user buttons are read and can be programmed for different functions as required by the user's application. Arduino and Raspberry Pi projects can also use the VP-EC-RDU when using the Arduino and Raspberry Pi serial ports as RS485.


The design is based on the PIC18F45K20 processor, has the ICSP port available for programming and product development, a RS485 port, etc.

Here are the schematics:

Here is the VP-EC-RDU in the enclosure with Graphic Overlay:



  • Low Power (9 - 24 VDC typical, max 28 VDC)
  • LCD Display 4 Line x 20 Character, White Characters, Blue Background (Other colours available upon request)
  • 2 Programmable Status LEDs  ( Green - Amber - Red )
  • Blue Power On Led
  • Audible (Programmable)
  • 4 User Pushbuttons  (Menu, Up, Down, Enter)
  • RS485 Interface 1 External RJ45 (Cat5e), 1 Internal RJ45 (Cat5e) , 1 Internal 4 Pt. Terminal Block
  • Programmable Baud Rates ( 9600 or 19.2K Baud N81 )
  • Easy Front Cover Removal (4 Screws), Rear Panel access cutout
  • Modbus ID Dipswitch Selectable
  • Modbus RTU Protocol
  • Compact, Rugged Case
  • 4x20 LCD Backlight Programmable  Always ON, Timed Off
  • Simple Operation Read and Write Modbus Holding Registers
  • Enclosure Dimensions 4" x 6" x 1.25"
  • Microchip PIC18F45K20 Processor c/w ICSP connection
  • Arduino and Raspberry Pi compatible when using RS485 with Arduino and Raspberry Pi projects

Two versions of this product are available:

Model: VP-EC-RDU is the complete unit with case, assembled and tested
Model: VP-EC-RDU-PCB is the same except without the case and graphic label.

MODBUS Functions

Modbus Function 02  Read Discrete Digital Inputs - User Pushbuttons

Address:  0

Data:   Menu      =   0x01

            UP          =   0x02
            DOWN    =   0x04
            ENTER   =   0x08             


Modbus Function 16  Write Multiple Holding Registers

The LCD uses both high and low bytes of the Holding registers and displays the ASCII characters as follows:

Holding Register Address  - Description

0 to 9        LCD Line 1  (20 Character ASCII String)
10 to 19    LCD Line 2  (20 Character ASCII String)
20 to 29    LCD Line 3  (20 Character ASCII String)
30 to 39    LCD Line 4  (20 Character ASCII String)
40             Status LED 1
41             Status LED 2
42             Audible                   0 = OFF, 1 = ON

Status LED States are defined as follows:

OFF                                        0x00
GREEN                                   0x01
GREEN SLOW FLASH           0x11
GREEN FAST FLASH            0x21

AMBER                                   0x02

AMBER SLOW FLASH           0x12
AMBER FAST FLASH            0x22

RED                                        0x04
RED SLOW FLASH                0x14
RED FAST FLASH                  0x24                     

Here are other views of the display:


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