I/O Scanning

Introducing I/O Scanning

The 171 CBU 98090 and 171 CBU 98091 processors includes an I/O Scanning service that exchanges register data with remote devices on the network over Modbus TCP. The service is enabled using the I/O Scanning configuration tab in Control Expert.

The I/O Scanning service supports the following Modbus function codes:

3: read registers
16: write registers
23: read/write registers

Use the I/O Scanning service when you want the processor to exchange data with a remote device at a fixed rate. Suitable applications include:

fast repetitive communications
applications that call for communication between one device and many remote devices, where different data is exchanged with different remote devices
applications that need to exchange data to more devices than is possible using the existing Modbus messaging function blocks
automatic event handling
controlling I/O devices

For non-cyclic communications, event-triggered actions, report generation, or notification of certain events (for example, excessive network or device load produced by data exchanges), use the Modbus Messaging service instead of I/O Scanning.

I/O Scanning Features

The processor includes the following I/O Scanning features:

Feature	Description
Number of I/O devices supported	0...32
Size of read per device	0...125 words
Size of write device	0...100 words
Maximum size of total read	500 words
Maximum size of total write	500 words
Maximum size of total read + write	512 words
Repetitive rate	10 ms...60000 ms
Application storage area	Non-contiguous %MW
Health status	1 bit per I/O scanning line entry

I/O Scanning Diagnostics

The I/O scanner monitors the health of Modbus communication, and stores health data in a block of two registers in the %IW memory area.

The control application can interrogate the health block as needed, and the application can confirm that communication with the remote device is healthy. If a communications interruption occurs, the application detects it and can respond based upon the needs of the application.

NOTE: You can also use the I/O Scanning web page to view the status of I/O Scanning.

Accessing I/O Scanning

To access the I/O Scanning tab:

Step	Action
1	In the Project Browser, navigate to the Project > Communication > Networks node.
2	Double-click the appropriate Ethernet network to open its configuration dialog.
3	In the Module Utilities field, select Yes in the IO Scanning menu.
4	Select the I/O Scanning tab to open the I/O Scanning configuration screen:

5	Enter the parameter settings under each of the column headings for one line of the I/O scanner configuration. Refer to I/O Scanning parameters (below).
6	Click the validate check box in the upper tool bar to confirm the I/O Scanning parameter settings.

I/O Scanning Parameters

The parameters above the table describe how the processor performs the scan:

Parameter	Description
Health Block (%IW)	Use the Health Block field to define the first word in a health table. The health table has a length of up to 2 words. Each bit in the table represents the status of a scanned module. The default setting is %IW1. NOTE: Modules are mapped to health words and bits, as indicated by the following examples: Line 1 is the 1st scanned module and maps to word 1, bit 0. Line 16 is the 16th scanned module, and maps to word 1, bit 15. Line 17 is the 17th scanned module, and maps to word 2, bit 0. Line 32 is the 32nd scanned module, and maps to word 2, bit 15.
Device Control Block	Use the Device Control Block field to disable I/O Scanning for a selected range of devices. When I/O Scanning is disabled, a master – for example a PLC – can assert control over an I/O slave device. When the check box is: De-selected (default): I/O scanning is enabled for devices appearing in the I/O scanning list. Selected: I/O scanning is disabled for devices in the range specified by the From and To IO scanning line numbers, as described in the Health Block description, above. NOTE: When I/O scanning is disabled for a device, the I/O scanner closes the connection to that device and sets the Health Bit to 0 (unhealthy) for that device.
Repetitive rate step	The Repetitive rate step is set in multiples of 10 ms (the minimum) through 200 ms (the maximum). The Repetitive rate (ms) column is where you enter a rate of time for how often you want the I/O scanner to send a query to the device after the rate has timed out. NOTE: The Repetitive rate (ms) of the I/O scanner table is a multiple of the rate displayed in the Repetitive rate step box. The real repetitive rate being executed by the I/O scanner service is shown in the Repetitive rate (ms) column. NOTE: If an entry in the Repetitive rate (ms) column is not a multiple of the Repetitive rate step, the entry is rounded up to the next multiple.

Parameter

Description

Health Block (%IW)

Use the Health Block field to define the first word in a health table. The health table has a length of up to 2 words. Each bit in the table represents the status of a scanned module. The default setting is %IW1.

NOTE: Modules are mapped to health words and bits, as indicated by the following examples:

Line 1 is the 1st scanned module and maps to word 1, bit 0.
Line 16 is the 16th scanned module, and maps to word 1, bit 15.
Line 17 is the 17th scanned module, and maps to word 2, bit 0.
Line 32 is the 32nd scanned module, and maps to word 2, bit 15.

Device Control Block

Use the Device Control Block field to disable I/O Scanning for a selected range of devices. When I/O Scanning is disabled, a master – for example a PLC – can assert control over an I/O slave device. When the check box is:

De-selected (default): I/O scanning is enabled for devices appearing in the I/O scanning list.
Selected: I/O scanning is disabled for devices in the range specified by the From and To IO scanning line numbers, as described in the Health Block description, above.

NOTE: When I/O scanning is disabled for a device, the I/O scanner closes the connection to that device and sets the Health Bit to 0 (unhealthy) for that device.

Repetitive rate step

The Repetitive rate step is set in multiples of 10 ms (the minimum) through 200 ms (the maximum).

The Repetitive rate (ms) column is where you enter a rate of time for how often you want the I/O scanner to send a query to the device after the rate has timed out.

NOTE: The Repetitive rate (ms) of the I/O scanner table is a multiple of the rate displayed in the Repetitive rate step box. The real repetitive rate being executed by the I/O scanner service is shown in the Repetitive rate (ms) column.

NOTE: If an entry in the Repetitive rate (ms) column is not a multiple of the Repetitive rate step, the entry is rounded up to the next multiple.

The parameters in the table describe the devices to be scanned:

Parameter	Description
Entry #	Valid range: 1 ... 32 Each entry represents an I/O Scanning exchange on the network.
IP address	The IP address of the scanned Ethernet slave device.
Device Name	To configure a device (Advantys island, DTM or PRA), click the ... button to open the Property dialog to start the device configuration software. NOTE: While the Property box is open, I/O Scanning cannot be edited.
Unit ID	The bridge index or slave device number when communicating from an Ethernet to another network bridging device. If you are using an Ethernet to Modbus bridge, this indicates the Modbus slave address number. If using an Ethernet to Modbus Plus bridge, this identifies the bridge index table entry: value range: 1 to 255 default value: 255
Slave Syntax	Use this drop-down menu to pick the way RD Ref Slave and WR Ref Slave values are displayed. The 4 choices are (with an example): Index: 100 Modbus: 400101 IEC 0: %MW100 IEC 1: %MW101
Health Timeout (ms)	This field sets the maximum interval between the responses from a remote device: range: 1 ms to 50 seconds interval: 1 ms After this time period expires, the received data is invalid. NOTE: Configure the health timeout setting to a longer time value than the repetitive rate.
Repetitive rate (ms)	The rate at which data will be scanned, from 10...60000 in multiples of the Repetitive rate step.
RD Master Object*	Destination address in the master PLC where, from each device, newly read information is stored. This parameter cannot be accessed. It is calculated automatically as the sum of: The From address (beginning) of Read ref. (in the zone above the table) The RD length value (in the table below) NOTE: Read and write master locations are non-contiguous.
RD Ref Slave**	Source address index in the slave/remote device
RD length	Number of words to read NOTE: If you enter a value greater than the maximum value of 125 words, Control Expert automatically applies the maximum value of 125 words, then creates an additional row or rows with the difference.
Last value (input)	This field configures the behavior of inputs in the event of a detected access error in relation to the remote device (for example: inoperative network or device power supply, and so on): Set to 0: fall back to 0 Hold last: maintain last value
WR Master Object*	Source address of the master PLC whose data is being written into the slave/remote device. This parameter cannot be accessed. It is calculated automatically as the sum of: The From address (beginning) of Write ref. (in the zone above the table) The WR length value (in the table below) Write operations are performed at the word level.
WR Ref Slave**	The address of the first word written into the slave/remote device.
WR length	Number of words to be written. NOTE: If you enter a value greater than the maximum value of 100 words, Control Expert automatically applies the maximum value of 100 words, then creates an additional row or rows with the difference.
Description	Additional information
Master refers to the client PLC that makes the request. *Slave refers to the server from which data is read or to which data is written.