The following Modbus functions are implemented on the TCP/IP, RS485 and USB ports:

• Function 3, Read Holding Registers
• Function 4, Read Input Registers
• Function 6, Write Single Register
• Function 16, Write Multiple Registers

Modbus register addressing is defined as 1 to 65536. In the low level comms the address offset from 1 is used. So this document refers to the Modbus Address (range 1 to 65536). Max Register Read or Write Length is 123. Treat all of the registers defined below as Holding registers.

Modbus allows more than one slave on a serial link, each slave is allocated a station address. A Modbus master can then read or write to a register in any attached slave. The GembusHub actually takes up 5 station addresses:

• 16 is the ident that would be used for accessing data relating to the GembusHub or ApStarnet as a whole
• 17 this is used for Gemstart link 1 data or ApStarnet port 1
• 18 this is used for Gemstart link 2 data or ApStarnet port 2
• 19 this is used for Gemstart link 3 data or ApStarnet port 3
• 20 this is used for Gemstart link 4 data or ApStarnet port 4

Some data within the device is related to the device or to all four Gemstart links. This data is normally accesses using station ident 16. It is organised in to three groups:

• Address 100 - GembusHub/ApStarnet General Settings
• Address 500 - GembusHub/ApStarnet Info
• Address 600 - GembusHub/ApStarnet Statistics

Each RS485 link has an identical data layout, but is accessed using a differnt staion ID. So reading a Modus register from station 17 would provide some information related to Gemstart/ApStarnet link 1, and reading the same register from station 20 would provide the same data but for Gemstart/ApStarnet link 4. This allows for an easy modular programming structure in an attched client PLC.

Many different data area are availibile here are some examples from the GembusHub version:

• Address 1000 - Link Settings
• Address 1100 - Link Statistics
• Address 1200 - Status Flags
• Address 1300 - Link Control Registers
• Address 2000 - Output Commands Array
• Address 3000 - Input Data Array
• Address 5000 - Configuration Data

The Gemstart input data area encompasses the real time Gemstart status monitoring block. Data is presented in a number of different maps, to allow the PLC designer to choice the best solution for there needs.

• Address 3000 - Full Gemstart Data map 800 registers
• Address 3800 - Only Status words, for all Gemstarts on Link 100 Registers
• Address 3900 - Only Load words, for all Gemstarts on Link 100 Registers
• Address 4000 - Only Load words, for all Gemstarts on Link 100 Registers
• Address 4100 - Status and Load words, for all Gemstarts on Link 200 Registers
• Address 4300 - Status, Load and Fault words, for all Gemstarts on Link 300 Registers

The output data area allows the control of each Gemstart. This is an array of 100 registers from mobus address 2000 onwards.

Much more data is availibile, group data and configuration data for example. For full details please refer to the Modbus data map, for either product, which can be downloaded from the support page.

Normally, we expect, this device would be used with one or more TCP/IP clients, but all of the data is avialable to a serial Modbus RTU master as well.