Modbus Compatible Device Driver

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The Modbus Compatible Device driver provides an interface to a Modicon PLC.

Driver Errors: To learn more about the cause of an error condition, refer to the Driver Summary Report and the Driver Error Report, both of which are available in the Reports page. The Show Stats button will also provide current and last error messages: Show Statistics Button Widget

Modbus drivers will commonly connect to TCP on port # 502, although variations may be found.

This driver supports multiple TCP/IP port connections between a single instance of the driver and the RTU.

Reference Notes:

Any of the following communication channels may be used:

Serial RTU
Serial ASCII
SA-85 I/O card for ModBus Plus communications
Embedded TCP/IP
Open Modbus TCP

This driver has the ability to save the last value written to each output tag, and to rewrite those values, either automatically when lost communications are restored, or manually by the press of a button. Carefully review the information in the Options tab to decide whether this feature should be used in your application. If this driver is being used in conjunction with a Driver Multiplexer, then configure the Driver Multiplexer to store the last values, not the drivers connected to the Multiplexer. In this case, only the Multiplexer should be configured to re-write automatically.

The number of bytes in a Modbus message is limited by the setting: ModiconVTSMaxBlock. By default, this is 125.

When this driver is used in combination with a TCP/IP tag, the standard TCP port is 502.

"Type specified" means addresses such as the HC1 and IR2 format. This does not apply to addresses written as numbers only. (For example, 40001 is always the first address in the 40000 block, and is equivalent to HR1 when ModiconTypedAddr1Offset is 0, or HR0 when ModiconTypedAddr1Offset is 1) If you change the property ModiconTypedAddr1Offset, it will be changed for all Modbus-compatible drivers in the application. If you need some drivers to be zero-based and others to be one based, you can achieve this by creating a parent Context tag for each driver, adding a property named ModiconTypedAddr1Offset to that Context tag. The child driver tag will take the value in the parent Context tag instead of the global property value. (See: Context tag properties, Settings tab)

Modbus Compatible Device Properties

Properties folder tab	Property	Value
Options tab	Comm Channel	Serial RTU
Serial tab	PLC Address	1
Serial tab	Time-Out Limit	0.5 (Make it shorter if Scan Interval is going to be very short)
Serial tab	Port	COM2 (name of Serial Port tag)
Serial tab	Retry Delay	0
Serial tab	Hold	deselected
Serial tab	Retries	2
Serial tab	RTS Key Off Delay	0

Related information:

Best Practices for Tags

Modbus Compatible Device I/O Addressing

Modbus Error Codes

Modbus Plus PLC Addressing

SCADAPack History Read

History Data for a TBox RTU

Koyo Address Translation

ModiconVTSMaxBlock

Modbus Compatible Device: ID tab

The ID tab of every tag includes the same common elements: Name, Area, Description, and Help ID.

Name:

Uniquely identifies each tag in the application. If the tag is a child of another, the parent names will be displayed in a separate area before the name field.

You may right-click on the tag's name to add or remove a conditional start expression.

Area

The area field is used to group similar tags together. By defining an area, you make it possible to:

Filter for particular tag groups when searching in the tag browser
Link dial-out alarm rosters to Alarm tags having a particular area
Limit the number of tags loaded upon startup.
Filter the alarm display to show only certain areas.
Filter tag selection by area when building reports

When working with Parent-Child tag structures, the area property of all child tags will automatically match the configured area of a parent. Naturally, you can change any tag's area as required. In the case of a child tag, the field background will turn yellow to indicate that you have applied an override. (Orange in the case of user-defined types. Refer to Configuration Field Colors)

To use the area field effectively, you might consider setting the same Area for each I/O driver and its related I/O tags to group all the tags representing the equipment processes installed at each I/O device. You might also consider naming the Area property for the physical location of the tag (i.e. a station or name of a landmark near the location of the I/O device). For serial port or Roster tags, you might configure the Area property according to the purpose of each tag, such as System or Communications.

You may define as many areas as you wish and you may leave the area blank for some tags (note that for Modem tags that are to be used with the Alarm Notification System, it is actually required that the area field be left blank).

To define a new area, type the name in the field. It will immediately be added. To use an existing area, use the drop-down list feature. Re-typing an existing area name is not recommended since a typo or misspelling will result in a second area being created.

There is no tool to remove an area name from VTScada since such a tool is unnecessary. An area definition will exist as long as any tag uses it and will stop existing when no tag uses it (following the next re-start).

Description

Tag names tend to be brief. The description field provides a way to give each tag a human-friendly note describing its purpose. While not mandatory, the description is highly recommended.

Tag descriptions are displayed in the tag browser, in the list of tags to be selected for a report and also on-screen when the operator holds the pointer over the tag’s widget. For installations that use the Alarm Notification System, the description will be spoken when identifying the tag that caused the alarm.

The description field will store up to 65,500 characters, but this will exceed the practical limits of what can be displayed on-screen.

This note is relevant only to those with a multilingual user interface:
When editing any textual parameter (description, area, engineering units...) always work in the phrase editor. Any changes made directly to the textual parameter will result in a new phrase being created rather than the existing phrase being changed.
In a unilingual application this makes no difference, but in a multilingual application it is regarded as poor practice.

Help Search Key

Used only by those who have created their own CHM-format context sensitive help files to accompany their application. Refer to Custom Help Files

Server List

Select (or create) a named server list. (Driver Server Lists) Servers for the list must be defined using the Application Configuration dialog, as described in Servers for Specific Services. Smaller sites that do not have multiple servers, or that use only the default server list, need not configure this field.

Modbus Compatible Device: Options tab

The Options tab of the Modbus Compatible Device tag properties folder can be used to set byte swapping for long integer, IEEE single or IEEE double bytes. If this driver is communicating over a modem, then the Options tab is also used to indicate that squelch noise should be suppressed, if present.

Modicon driver tag - options tab

Comm Channel

Select the communication channel and protocol to use for this Modbus Compatible Device driver. This will be one of:

Serial RTU Select to use the RTU mode of the Modbus serial communications standard.
Serial ASCII Select to use the ASCII mode of the Modbus serial communications standard.
SA-85 (MB+) Select to use the Modbus Plus communications protocol via a Modicon SA-85 card.
Embedded TCP/IP Select to use the Embedded TCP/IP protocol.
Open Modbus TCP Select to use the Modicon Open Modbus/TCP specification to receive Modbus protocol over Ethernet TCP/IP.

Squelch Noise Present

Applies when communicating over a modem. Select this box to enable the squelching of background noise. By default, the Squelch Noise Present check box is not selected.

Reverse Long Integer Byte

Select whether byte swapping should occur for long integer values. By default, the Reverse Long Integer Byte check box is not selected.

Reverse IEEE Single Byte

Select whether byte swapping should occur for IEEE single byte values. By default, the Reverse IEEE Single Byte check box is not selected.

Reverse IEEE Double Byte

Select whether byte swapping should occur for IEEE double byte values. By default, the Reverse IEEE Double Byte check box is not selected.

Force Multi Element Write OpCodes

Certain hardware requires data to be written using multi-element OpCodes (15 and 16) even though the actual data is single-element (OpCodes 5 & 6). By selecting this box, you can ensure that this driver instance is able to communicate with devices that require the multi-element codes.

Store Last Output Values

When selected, the driver will maintain a record of the last value written to each output address. This may be useful in at least two situations:

For hardware that does not maintain its state during a power loss and must be restored to that state when re-started.
When failed hardware is replaced by a new device and you would like to start that device with the values last written to the old one.

If the last output values are stored, they may be re-written by either of two methods:

Automatically, when communication is restored to the device.
Manually by way of a button press. See, Rewrite Outputs Widget for details.

Changing this value from selected to deselected will cause all stored values to be erased immediately.

If this driver is being used in conjunction with a Driver Multiplexer, then configure the Driver Multiplexer to store the last values, not the drivers connected to the Multiplexer. In this case, only the Multiplexer should be configured to re-write automatically.

Enable Auto Rewrite

If selected, the Store Last Output Values option will also be activated. This option causes the driver to rewrite the last value written to each output, in the event that communications are lost and then restored.

Use this option only if you are certain that you want the last values to be rewritten automatically after an interruption in driver communications.

Maximum Coalesce Gap

Use only if the Modbus compatible device does not support reads from addresses that are not explicitly declared in the device. VTScada will attempt to coalesce reads from a range of addresses, which may lead to an error with these devices.

If left blank, there is no effect on driver coalescing.

Set to zero to allow for devices that cannot read from addresses that are not explicitly declared.

Set to a number greater than zero to fine-tune read coalescing. The gap is measured in units of two-byte words. Warning: when using low maximum coalesce gap values, it is important to avoid unnecessary gaps in VTScada tag addresses.

if this value is set, driver response time may be slowed due to the increased number of read blocks.