Beijer Electronics NA-9379 - Manual (Page 59)

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MODBUS Programmable I/O NA-9379 FnIO S-Series
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A.2 MODBUS Transmission Mode
Two different serial transmission modes are defined: The RTU mode and the ASCII mode. It defines the bit
contents of message fields transmitted serially on the line. It determines how information is packed into the
message fields and decoded.
A.2.1. RTU Transmission Mode
When devices communicate on a MODBUS serial line using the RTU (Remote Terminal Unit) mode, each 8–bit
byte in a message contains two 4–bit hexadecimal characters. The main advantage of this mode is that its greater
character density allows better data throughput than ASCII mode for the same baudrate. Each message must be
transmitted in a continuous stream of characters.
Start Address Function Data CRC Check End
≥ 3.5 chars 1 char 1 char Up to 252 chars 2 chars ≥ 3.5 chars
A.2.2. ASCII Transmission Mode
When devices are setup to communicate on a MODBUS serial line using ASCII (American Standard Code for
Information Interchange) mode, each 8–bit byte in a message is sent as two ASCII characters. This mode is used
when the physical communication link or the capabilities of the device does not allow the conformance with RTU
mode requirement regarding timers management.
Start
Address Function Data CRC Check End
1 char 2 chars 2 chars Up to 252 chars 2 chars 2 chars CR,LF
A.3 Supported MODBUS Function Codes
Function Code Function Description Unicast / Broadcast
1 (0x01) Read Coils Read output bit Unicast
2 (0x02) Read Discrete Inputs Read input bit Unicast
3 (0x03) Read Holding Registers Read output word Unicast
4 (0x04) Read Input Registers Read input word Unicast
5 (0x05) Write Single Coil Write one bit output Unicast / Broadcast
6 (0x06) Write Single Register Write one word output Unicast / Broadcast
8 (0x08) Diagnostics (Serial Line only) Read diagnostic register Unicast
15 (0x0F) Write Multiple Coils Write a number of output bits Unicast / Broadcast
16 (0x10) Write Multiple registers Write a number of output words Unicast / Broadcast
23 (0x17) Read / Write Multiple register
Read a number of input words /
Write a number of output words
Unicast
- Refer to MODBUS APPLICATION PROTOCOL SPECIFICATION V1.1a