Ignition Allen-Bradley Logix driver: ControlLogix, CompactLogix, slot, connection path and tag groups

Ignition reads ControlLogix and CompactLogix controllers through the Allen-Bradley Logix driver, part of the Allen-Bradley Ethernet module of its OPC UA server. It connects with little more than an IP address and browses the controller's tags by name. This article covers the settings, connection paths, tag groups and diagnostics from the Ignition 8.1 and 8.3 user manuals, and notes where the two differ. What Ignition is: What is Ignition.

1. Which driver for which controller

The 8.3 manual lists these drivers in the Allen-Bradley Ethernet module:

The Logix driver page notes that the driver is optimised for firmware version 21 and later and works with earlier firmware at much lower performance. Both versions of the manual say the Allen-Bradley drivers officially support firmware version 16 and later; older firmware may work but is unsupported, and the manual suggests another OPC server for it. Direct connections to RSLinx Emulate 5000 are not supported. One difference: the 8.1 overview lists MicroLogix 1100, 1200, 1400 and 1500, while the 8.3 MicroLogix page says the 1200 and 1500 are not fully supported and cannot be browsed.

2. Creating the device connection

In 8.3 the connection is made on the Gateway webpage under Connections > Devices > Connections: Create Device Connection, choose Allen-Bradley Logix Driver, enter a name and the hostname, and create it. In 8.1 the page is Config > OPC UA > Device Connections, Create new Device. In both, the new device shows Disconnected first and then Connected. The 8.3 page adds that the driver needs the OPC UA module installed and enabled, otherwise the device faults with a Missing Dependency error.

The device name appears in the OPC item path of every tag read through it, so we settle it before tags are created. In 8.1 it must be alphanumeric; in 8.3 it must start with a letter or an underscore and contain no special characters.

3. The settings and what they mean

The defaults in the last column are those printed on the 8.1 page; the 8.3 page describes the same settings and prints only the port default.

In 8.3, Slot Number and Max Concurrent Requests sit under Advanced instead of Connectivity.

4. Slot number or connection path

The Logix driver describes the hostname as the Ethernet module to route through and Slot Number as the controller's slot, counted from 0. The overview pages add that a connection path is required when the processor is reached through a bridge device or an external Ethernet module such as an ENBT or EN2T, for example through what the manual calls a ControlLogix Gateway. The hostname then points to the Ethernet module of the bridge chassis, and the driver is chosen for the target controller.

The path is built in pairs, one leaving a module and one entering the next:

• It starts with 1, out to the backplane. The slot of the first Ethernet module is not entered.
• Then the slot of the module to move to, and the port to leave it by: on an ENET, ENBT or EN2T, 2 is the Ethernet port; on a CNB, 2 is the ControlNet port; on a DHRIO, 2 is DH+ channel A and 3 is channel B. In every module, 1 is the backplane.
• Then the address of the next entry point: an IP address, a ControlNet address, or a DH+ station number in octal from 0 to 77.
• For a Logix controller the path ends with 1 and the processor slot.

A valid path always has an even number of entries; an odd count means a missing step. The 8.3 example 1,3,2,192.168.0.56,1,0 reads: out to the backplane, to the ENBT in slot 3, out of its Ethernet port, into the module at 192.168.0.56, out to that chassis backplane, to the processor in slot 0.

5. Browsing tags and how they appear

All Allen-Bradley drivers use tag-based (symbolic) addressing: Ignition browses the controller for its tags. In the 8.3 Designer, Add > Browse Devices in the Tag Browser opens the Connected Devices window, where tags are selected and added to the tag provider. When an item is dragged in, the characters ! @ # $ ^ & * + [ ] in its name become underscores.

What the driver pages say about the browsed tags:

• A BOOL[64] in the controller appears as DWORD[2], so element 32 becomes bit 0 of the second word.
• The Logix driver also reads DT, LDT, LTIME, TIME and TIME32 types (new in 8.1.28).
• Allen-Bradley does not support reading members of an Add-On Instruction nested in another AOI, and such tags cannot be browsed in RSLogix and RSLinx; the manual says alias tags can read and write them.
• The ControlLogix (Legacy) page adds that a tag with External Access set to None is not returned; it must be Read Only or Read/Write in RSLogix 5000.

If Rockwell's own software cannot see the controller either, see RSLinx or FactoryTalk Linx cannot see the controller.

6. Tag groups and the load on the controller

The tag group sets how often a tag is read. Direct runs at one fixed rate and is the mode of the default tag group; Driven switches between two rates on a condition; Leased reads tags shown on an open window or view at the Leased/Driven Rate and the rest at the base Rate. Data Mode chooses subscribed reads, which reduce traffic and fall back to polling if the subscription fails, or polled reads, which give a predictable load. The manual notes that highly optimised systems generally use several tag groups, so each value is read only as fast as it needs to be, for example 500 ms for some and 30 seconds for others.

The driver packs the items into requests, with a number of items per request that depends mainly on the protocol, and every request waits in a queue before it runs.

7. Device status and diagnostics

The Gateway device list has a Status column that should show Connected. Each device connection also has diagnostic tags: Connected (a Boolean), State (for example Connected or Disconnected), Status (more detailed than State), Hostname, Name and Port. A Sampling folder holds an Aggregate folder and one folder per sample rate in use.

The tag to watch is OverloadFactor: 100 times the average time requests spend in the queue, divided by the actual sampling interval of the group (the 8.1 page's worked example uses the configured interval). Above 100 %, requests are read slower than the configured rate. The 8.1 page says this metric replaced Load Factor in 8.1.6 and works an example: a 250 ms group whose requests wait 4.24 ms gives about 2 %. In 8.3, View Details on a device shows the request count, throughput, mean response time, a response time chart and the device log.

What TDL checks first

These are our own working habits before connecting Ignition to an Allen-Bradley controller, not Inductive Automation requirements:

• We check the controller firmware revision, which decides between the Logix driver and a legacy driver.
• We check how the controller is reached: the module behind the hostname, the controller's slot and every bridge on the way, so the path can be written pair by pair.
• We check the connection capacity of the Ethernet module. Rockwell's technical data 1756-TD003R lists 256 CIP and 128 TCP connections for a 1756-EN2T and 128 CIP and 64 TCP for a 1756-ENBT, and states that CIP connections can serve explicit or implicit applications in any combination. We list what already connects through the module, such as HMIs, other SCADA and messages. More on module limits: 1756-ENBT end of life.
• We check which tags need a fast rate, set the tag groups from that, and watch OverloadFactor and the mean response time after start-up.
• We check External Access and nested AOIs on the tags the screens need.

How TDL approaches it

TDL Industrial Automation is an Ignition Certified Integrator. Most plants we work in run Allen-Bradley controllers, so the work starts from the PLC programs and network drawings already on site. If the plant is still choosing a platform, see Legacy HMI/SCADA migration paths. More on these services: Ignition SCADA integrator in Vietnam and Connecting existing PLCs and equipment to SCADA.