Section 6 - Communication
6.3.2 Modbus Server
The recorder supports ModbusTM RTU (Remote Terminal Unit) over Ethernet and optional RS485 interfaces. An RS485 interface will support up to 64 Data Recorders linked to a single computer. Each of the Recorders will be identified by a unique unit address (programmed under System Settings - Modbus). This address is set at 1 to 256 and must be different from any other recorder on the loop. Similarly if using the RS485 serial port the baud rate must be set to match the system – 9600 to 115,200 baud.
Modbus over Ethernet is supported through Port 502.
Note that Modbus ASCII is not supported and this recorder supports a minimal subset of registers and is read only. The functions included in the subset are
Function 3 - Read Holding Registers
Function 4 - Read Input Registers
All Modbus registers are 2 bytes = 1 word.
The physical connection details are shown in Section 2 – Isolated Serial Option.
6.3.2.1Modbus Status Registers
The Status Registers Are 16 bit Integers in the form of two bytes as follows (Low Byte = Bits 0 through 7).
| Bit 15 - 8 |
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| Bit 5 |
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| Bit 1 |
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| Not |
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| 0 or 1 |
| 0 or 1 |
| 0 or 1 |
| 0 or 1 |
| 0 or 1 |
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When the bit is a “1” the condition is true. There is one register for each point or channel and the registers start at number 3056. So Register no. 3056 is Channel 1 status, Register 3057 is Channel 2 status through Register 3071 which is Channel 18 status. These Registers are read only and are accessed using Function 4.
6.3.2.2Modbus Data Registers
The Data Registers hold the current floating point value for each of the 18 data points (Channels) as IEEE floating point numbers which require 4 bytes (2 words) as shown below. They are in the format
| Sign Bit | Exponent (+127 biased) | Mantissa (Extra implied 1 bit) |
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| 1 bit | 8 bits | 23 bits |
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| Byte 0 |
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SEEEEEEE |
| EMMMMMMM | MMMMMMMM |
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The following tables show the mapping of the recorder parameters into Modbus registers. Knowledge of these Modbus register assignments is only needed to make use of third party communications software. Both registers – high and low, must be accessed together to ensure valid data.
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