Technical White Paper
This white paper describes the specifications for the Modbus protocol implementation in the Neon Remote Logger (NRL). The Modbus protocol implemented in the NRL is a partial implementation of the full Modbus protocol.
Modbus is a serial communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Modbus has grown to a widely adopted de facto standard in the Industrial Measurement sector and is now the most common protocol for industrial measurement applications. It is implemented in the NRL as a partial implementation of the full Modbus protocol.
The Unidata approach to this partial implementation is to provide only two NRL functions
- Function 1 is to extract (get) data from specified registers within a Modbus RTU
- Function 2 is to place data (put) into a specified register within a Modbus RTU
The specific register information and its corresponding encoding and decoding information required for interpretation are defined when the NRT logging scheme is created.
There are three ways that this register information can be set up using the NRT logger support software, Starlog V4.
Generic Modbus Instrument schemes
Large Modbus Builder schemes
Modbus TCP Server
NRT Communications Systems
Cellular NRTs
Inmarsat Satellite NRTs
The Inmarsat satellite system uses GEO (geostationary) satellites that are always available.
This implies low written Modbus register latency, limited only by the chosen communications frequency with the Neon Server.
Globalstar Satellite NRTs
NRT Modbus Implementation
- Modbus Master – Basic Implementation
- RTU Transmission Mode
- Baud rate: 1200, 2400, 4800, 9600, 19200 (CDT adjustable)
- Parity: 8,n,1; 8,e,1; 8,o,1; 8,n,2 (CDT adjustable)
- No Line Termination provided
- No Line Polarisation/Biasing required (or provided)
- Request Timeout: 250 ms (CDT adjustable)
- Failed Request – number of retries: 3 (CDT adjustable)
- Frame turnaround delay: 3.5 chars (fixed in driver code)
- Buffer size: 20 bytes (fixed in Modbus code)
- RS-485 3V (meets specs, could be 5V)
NRT Modbus Connections
Using the Generic Modbus Instrument in Starlog V4
RTU Address = Address number of the RTU on the BUS (1..247) Note: Addr = 0 is the broadcast address and is supported by the NRT
Various sampling interval methods are available.
- Log Interval Modbus registers are read at the scheme log interval
- Fixed Interval Modbus registers are read every “n” seconds
- Continuous Modbus registers are read at the scheme Scan interval
- Manual Modbus register reads are triggered by scheme Events
Refresh Rate (Fixed Interval only). A number of seconds before the next Scheme Log Interval. This defines the number of seconds between each Modbus interrogation so that the RTU readings can be collected and placed into the Logger Channels.
WARNING: The Refresh Rate must be modulo scan rate. i.e. if the scan rate is 5 secs then the Refresh Rate must be in units of 5 secs, otherwise, NRT MODBUS will not activate.
New Modbus registers are added to the scheme using the “Add” button.
Scheme memory limits the number of registers to a total of 70 bytes, providing 35 registers if integers are used or around 15 registers if they are floats or a mixture of the two.
Using the Modbus Builder Instrument in Starlog V4
This option does not allow for Writes, however, it allows for interrogation of an arbitrary number of (hundreds of) Modbus registers by a Starlog V4 scheme.
The Modbus Builder Instrument uses a wizard to configure the instrument.
A CSV (Comma Separated Variable) text file lists and configures each Modbus register in the scheme. Each line of the CSV file configures and individual Modbus register using the following fields.
DESCRIPTION,TAG NAME,MODBUS ADDRESS,TYPE,ENG UNITS,READ / WRITE
The meaning of each parameter is as follows.
- Description – A textual description of the Modbus register’s function
- TAG – The exported Modbus register TAG as it appears on the Neon Servers
- Modbus Address – Address number of the RTU on the BUS (1..247)
- Type Modbus – register type. One of {REAL, INT or BOOL}
- Eng Units – The register’s Engineering Units. E.g. kPa, degC, Volts, etc
- Read/Write Modbus writes are currently not supported by this instrument
The second wizard screen specifies the Modbus data conversion method to be applied to the received data and the Modbus address mappings for each Modbus function.
Using the Modbus TCP Server Interface
The Neon Server provides a Modbus TCP Server (Slave) interface that may be written to directly by a Modbus Master. E.g. DeltaV
A standard Generic Modbus Instrument scheme must be operating on the NRT. The logger’s Node Type must be set as “Modbus Server” on the Neon Server.
The Neon Server uses the list of data channels in the logger’s scheme to pass Modbus register information to the Modbus TCP Server interface.
When polled by a Modbus Master, the Modbus TCP Server interface immediately returns the last value received from the NRT in the data channel. Data channel values are updated as and when the NRT communicates with the Neon Server according to the NRT’s Communications Frequency.
Modbus Register values written to the Modbus TCP Server interface are transmitted as custom commands to the logger when it next communicates with the Neon Server according to the NRT’s Communications Frequency.
Scheme memory limits the number of registers to a total of 70 bytes, providing 35 registers if integers are used or around 15 registers if they are floats or a mixture of the two.
