Using Modbus Server

Modbus Register Display

Modbus registers are displayed in bottom half of the window.
The register address (integer format) is displayed in the left hand column, the corresponding value contained in that register is displayed in the right hand column.
In Modbus Server Pro version the value can be modified by double clicking and entering a new value. The new register value is updated in the Modbus memory upon leaving the field by pressing Enter or Tab. Permissible values range from -32767 to +32768.
Add Address

Modbus supports four different memory types:
  • Holding - 16 bit register
  • Input - 16 bit register
  • Digital Input - single bit
  • Digital Output - single bit
The type of memory being displayed is selected via radio buttons:
Add Address
Please note that all four memory types are maintained simultaneously and separately and can be queried by clients/masters regardless of its display status.

Each register value is displayed in a format selectable via the format selector radio buttons above the display.
Add Address

Adding Registers

Registers are added automatically when a TCP client or RTU master reads or writes register content. The initial value of automatically added registers is always 0. (This may be changed in a future version).

New register addresses can also be added manually via Apple Command-A keyboard shortcut.
Add Address

Access Indicator

Access Indicator
The Modbus access indicator changes colour on read/write access via Modbus but not when register values are changed by user interface. The indicator will flash blue for read access and orange for write access.

Saving Register contents

Apple Command-S and Shift-Apple Command-S will save (Save As) the current register configuration and the register contents. This can be useful for emulating a particular device or maintining information between sessions.
Files are saved with "mbr" as an extension. The File Open command (Apple Command-O) is used to load a previous register configuration into the Modbus server.

Error Simulation (Pro version)

Note: Although ModbusServer (Basic not Pro) only supports one option ( No Error ), it will still respond correctly to errors in the received modbus messages.

Error Simulation
This selection will force the server into replying with an error code. This function may be used to test the error response of client.
Error description:
  • Server Busy - the server can not reply at this time
  • Illegal Function - the requested function code is not understood by the server
  • Illegal Address - a register address in the request is out of range
  • Illegal Value - the data value in the request is illegal (out of range)

Mute function

Mute Button
By activating the mute function the Modbus TCP server and RTU Slave will continue to accept connections and data but will not send a reply to the client.
This function is useful for testing clients response to timeout errors.

Viewing Raw Modbus Data

Command-D (or Window->Toggle Data Log) will show the data log window where all raw data In and Out of the Modbus server is displayed. The data is formatted in Hexadecimal and is displayed with a timestamp and Rx/Tx indicator.
Functions and Registers TCP Server