rfIDeas Modbus TCP Register Specification :

In general, the data for the Modbus TCP reader interface is very similar to the EtherNet/IP reader interface. The only real difference is byte ordering within the 16-bit Modbus registers. 

Output Data

Modbus Holding Register Addresses 0 & 1 (2 registers) 

Holding Register 0: LED Control, only LSB is significant; MSB is ignored

• 0 (0x00) = Off – Turn off the LED 
• 1 (0x01) = Red – Illuminate the Red LED 
• 2 (0x02) = Green – Illuminate the Green LED 
• 3(0x03) = Amber – Illuminate both LEDs for Amber 
• 255(0xFF) = Allow the reader to automatically control the LED (Red – no card data, Green – valid card data received) 

Holding Register 1: Beep Now, only LSB is significant; MSB is ignored

• 1 (0x01) = Short Beep – A short beep is played 
• 2(0x02) = Two Short Beeps – a short beep is repeated twice 
• 3(0x03) = Three Short Beeps – a short beep is repeated 3 times 
• 4(0x04) = Four Short Beeps – a short beep is repeated 4 times 
• 5(0x05) = Five Short Beeps – a short beep is repeated 5 times 
• 101(0x65) = One Long Beep – a Long beep is played 
• 102(0x66) = Two Long Beeps – a long beep is repeated twice 

Input Data

Modbus Input Register Addresses 0-18 (19 registers) 

Input Register0: Sequence Number

• The number is incremented to indicate a change in ID data and Bit
  Count. 

Input Register 1: LUID

• Logical Unit ID – Unique Identifier for the Reader (if assigned
  during reader configuration) 

Input Register 2: ID Bit Count / Present

• In Polling Mode, this value is the total bit count of the incoming
  ID. In Listen Mode, this value is 0 when a card is not present, and 1 when a card is present.  

Input Registers 3-18: Card ID (16 registers, 256 bits)

• Raw card ID data where bits are “right-aligned”, therefore, the
  least significant bit is contained in the least significant bit of Register 18.
  For example, if a card contains 24 bits of data 0xB7748A, the Modbus Registers will contain the following data: