3.3 Modbus items configuration
The configuration dialog is shown in Figure 3.5.
The lower part of the configuration dialog displays the configured signals. Each line has a symbolic name and corresponds to one signal or a group of signals. New item can be added by pressing the Add item button in the right column of the configuration dialog. The item can be similarly edited by pressing the Edit item button. Some of the parameters (Name, Item Address, Type and Count) can be edited directly in the Item list. If you choose Advanced mode it unlocks all other parameters for direct edit in the Item list.
The individual columns have the following meaning:
- Name
- – A unique signal name.
- Slave
- – Select slave device. Available only in Modbus TCP/IP Master item configuration.
- Slave Address
- – Slave device address. Available only for Modbus RTU Master item configuration.
- Item Address
- – Address of the object in the device. All objects (values) within one station have an identification number (address) from the range 0 to 65535.
- Type
- – Signal type. The names respect the Modbus specification:
Input Digital input Coil Digital output Register (16bit) integer number 0…65535, Register (32bit) integer number 0…4294967296, Register (float) 4-byte floating point number Register (double) 8-byte floating point number - Count
- – Number of values. One item can represent a group of values which are read at once using the multi-input/output block. This number defines how many values to read. The number does not necessarily correlate with the number of inputs/outputs of the block. In such a case, the unused pins are not updated.
- Inhibit Time
- – Valid only for Writable (W) items. Defines the minimum length of time that must be allowed to elapse between the transmissions of the item.
- Period
- – Refresh period in seconds. Nonzero values define the period, zero respects the period given by the corresponding IODRV block.
- Initial value
- – The initial value to set the signal to when initializing the driver. Use square brackets for groups of values. Separate the values by spaces.
- Readable (R)
- – Tick this checkbox to allow reading of the value in the REX algorithm. In Slave configuration, such items will be writable for the Master.
- Writable (W)
- – Tick this checkbox to allow writing of the value from the REX algorithm. In Slave configuration, such items will be readable for the Master.
- Passive (P)
- – In some cases it is more efficient to transmit a large group of signals at once (maximum is 125 values). But we still want to work with individual signals or less populated groups in the algorithm. In that case we define one big array and a number of small groups overlaying the data registers. Only the big array is active, the other signals are set to passive. This field is available only in the Master configuration.
- Join next (J)
- – Tick this checkbox to use the read-write command of the Modbus specification. The readable signal is joined with its successor (the writable item one line below) and the data is exchanged using a single command. This field is available only in the Master configuration.
- Multi command (m)
- – Tick this checkbox to force multi-read or multi-write command even if only one value is transmitted. Useful for Modbus stations supporting only a subset of Modbus commands. This field is available only in the Master configuration.
- Swap word (a)
- – 32-bit numbers are represented by 2 successive 16-bit numbers in Modbus. This flag defines the order of lower and upper word. The same holds for real numbers as they are represented by 2 or 4 successive 16-bit registers.
- Swap byte (b)
- – The upper byte of the 16-bit number is transmitted first in Modbus (the so-called big-endian format). However, some devices use little-endian implementation. This flag becomes handy in such a case.
- Is Signed (S)
- – Tick this checkbox to treat the integer number as a signed number.
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