AVEVA™ Plant SCADA

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Last UpdatedDec 03, 2025
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Displays information about I/O devices in a Plant SCADA system. This information is only displayed if the Plant SCADA computer is configured as an I/O server.

The page presents information for the I/O devices connected directly to the I/O server, as well as any devices that are configured as redundant peers to these local devices. This can include I/O devices that are connected to a different I/O server.

For I/O devices that are connected directly to the I/O server, a full set of fields is presented. This will include values retrieved from the project configuration, as well as dynamic information such as the unit online state, read response times, active errors and tag subscriptions.

For redundant peers connected to a remote I/O server, only fields retrieved from the project configuration will be available. The values displayed will reflect those detected at the time a project was compiled.

To access a full set of diagnostics for any I/O devices that are redundant peers, refer to the Kernel on the I/O server to which they are connected.

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Use the Page Up and Page Down keys to scan the I/O devices for this I/O server. To view redundant I/O devices on other I/O servers, use Verbose mode. The following tables include the fields displayed when Verbose mode is active.

I/O Device Information

Unit	The name of the I/O device defined in the project.
IO Server	The name of the I/O server that is servicing this I/O device.
Comment	A description of the I/O device.
Unit Network No	The I/O device number defined in the project.
PLC Number	The physical I/O device address defined in the project.
Port Name	The communication port to which the I/O device is connected.
Protocol	The protocol used for communication with the I/O device.
Primary	Indicates if the I/O device is configured to be in primary mode. Yes = Primary No = Standby. If the I/O device is in primary mode, the server starts a communication channel with the I/O device as soon as the server is activated. If an I/O device is in standby mode, the I/O device remains inactive when the server starts (until a primary I/O device becomes inoperative).
Priority	The relative priority configured for this I/O device. This field is only relevant for redundant I/O devices. The standby I/O device with the highest priority will take over from the primary when required, the next highest priority will take over from that device if required. Be aware that the highest priority will have the lowest priority value.
Cached	This field indicates if the I/O device data is cached. Caching may impact communications with the physical device.
Cache Timeout	If the I/O device is cached, this field displays the cache timeout value. Data is held in the cache for this timeout period before being discarded and re-read from the I/O device. Only read data is cached.
Read Only Mode	Indicates whether the device has been set to read-only. If it is set to TRUE, any attempt to write to a tag associated with the I/O device will be unsuccessful.
Exclusive Mode	Indicates if an I/O device has been set to exclusive mode. If set to TRUE, only one I/O device (on one I/O server) will activate communications to the physical device at a time. The best available I/O device (online with the lowest priority) at a particular time will be chosen to actively communicate. When this I/O device is no longer the best available, it will deactivate, which will in turn trigger the best available I/O device on another server to activate. If set to FALSE, the best available I/O device will typically be the only active I/O device; however this may not be enforced. When a different I/O device becomes the best it will activate immediately, without waiting for the other I/O device to deactivate. An I/O device that is not the best may also be forced into actively communicating with the physical device if a particular client can only communicate with the I/O server that I/O device is running on.

The following fields are not shown for I/O devices that reside on a peer I/O server.

Unit Status	The status of the I/O device. Only I/O devices that are serviced by this I/O Server are shown, and their redundant I/O devices if page is shown in Verbose mode. The unit status can be one of the following: RUNNING - Indicates that the communication link with the I/O device is good. STANDBY - Indicates that the communication link with the I/O device is good, but communication with that I/O device is currently being performed by another port. This port is in standby mode. STARTING - Indicates that the server is currently establishing a communication link (with the I/O device). STOPPING - Indicates that the server is currently relinquishing control of the communication link (with the I/O device). OFFLINE - Indicates that the server cannot establish a communication channel with the I/O device. If a standby port or server is available, Plant SCADA tries to communicate to the I/O device using that port. REMOTE - Indicates that the status of the I/O device is OK, but it is not currently connected.
Subs State	Indicates if an I/O device is currently responsible for providing updates for client subscriptions. This will be represented by one of the following: Global Active - this is the best available I/O device and is the preference for all client subscriptions. It is responsible for providing updates to all of these subscribed clients. Forced Active - this is the preferred redundant I/O device for clients that are unable to reach the Global Active I/O device. It will only provide updates to clients that are unable to contact the Global Active I/O device. Local Active - this is used for a redundant I/O device that is the best available on its I/O server (but not across all I/O servers). It will transition to Forced Active if a subscription is received from a client. The I/O device is not yet providing updates to any clients. Inactive - this is used for I/O devices that are unavailable or have a peer redundant I/O device on the same I/O server in one of the above states. The I/O device is not providing updates to any clients.
Active subscriptions	If the Subs State field indicates an I/O device is either Global Active or Forced Active, this field will indicate the number of client subscriptions that are currently active.
Blocking Constant	The current blocking constant value for this I/O device, as specified in the protocol. This value influences how requested I/O is blocked together.
Read Response Times	The time taken by the driver to process read and write requests. This represents the time taken to process a single read or write operation to the I/O device. This time depends only on the physical response time of the I/O device, because no queue waiting time is included. This field reflects any tuning of the communication channel (for example doubling the baud rate will half the response time). The following values are displayed: Average Minimum Maximum Count. Note: One I/O device with a slow response may slow down your entire system. For example, if you have an I/O device with a response of 2000 ms, any pages in your system that use data from that device, will have a minimum update time of 2000 ms.
Generic Error	The last generic error code returned by the driver. Because most protocol drivers have their own special errors, they cannot be recognized by the I/O server. The drivers convert their special errors into generic errors that can be identified by the server.
Error Handle	This field is for internal diagnostics.
Driver Error	The driver-specific error code. Each driver has its own special error codes. Refer to the driver specific errors (for the particular protocol) for an explanation of each of the error codes.
Error Message	The alert message associated with the generic error code.
Error Count	The total number of errors from the I/O device.
Restarts	The number of times the server has tried to establish a connection with the I/O device. This number is normally 1, because the server establishes a connection at startup. If this field displays a number greater than 1, there is a problem with the communication channel.

Memory Mode Device Information

The following fields are shown when the device is functioning in memory mode.

Unit	The name of the I/O device defined in the project.
IO Server	The name of the I/O server that is servicing this I/O device.
Comment	A description of the I/O device.
Unit Network No	The I/O device number defined in the project.
PLC Number	The physical I/O device address defined in the project.
Port Name	The communication port to which the I/O device is connected.
Protocol	The protocol used for communication with the I/O device.
Primary	Indicates if the I/O device is configured to be in primary mode. Yes = Primary No = Standby. If the I/O device is in primary mode, the server starts a communication channel with the I/O device as soon as the server is activated. If an I/O device is in standby mode, the I/O device remains inactive when the server starts (until a primary I/O device becomes inoperative).
Priority	The relative priority configured for this I/O device. This field is only relevant for redundant I/O devices. The standby I/O device with the highest priority will take over from the primary when required, the next highest priority will take over from that device if required. Be aware that the highest priority will have the lowest priority value.
Unit Online Status	For devices in memory mode, this will always display "N/A".

Dial-up Connection Devices

The following fields are shown when the I/O device is not in memory mode and configured for scheduled dial-up communication.

Dial-up Connection	The status and history of the dial-up connection. SUCCESS - The number of successful dial-up attempts. FAIL - The number of unsuccessful dial-up attempts. TOTAL - The total number of dial-up attempts. NEXT - The time of the next scheduled dial-up attempt.
Total Connect Time	Accumulated time over all connections.
Average Connect Time	Average time per connection.
Subs State	Indicates if an I/O device is currently responsible for providing updates for client subscriptions. This will be represented by one of the following: Global Active - this is the best available I/O device and is the preference for all client subscriptions. It is responsible for providing updates to all of these subscribed clients. Forced Active - this is the preferred redundant I/O device for clients that are unable to reach the Global Active I/O device. It will only provide updates to clients that are unable to contact the Global Active I/O device. Local Active - this is used for a redundant I/O device that is the best available on its I/O server (but not across all I/O servers). It will transition to Forced Active if a subscription is received from a client. The I/O device is not yet providing updates to any clients. Inactive - this is used for I/O devices that are unavailable or have a peer redundant I/O device on the same I/O server in one of the above states. The I/O device is not providing updates to any clients.
Active subscriptions	If the Subs State field indicates an I/O device is either Global Active or Forced Active, this field will indicate the number of client subscriptions that are currently active.
Poll Reads Total	The accumulated number of poll reads issued to physical device to service tag subscriptions.

AVEVA™ Plant SCADA

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