An Ignition 8.x OPC UA server has a default ceiling of 100 active client sessions [S2][S3], while the supplied Siemens material discusses WinCC Unified as an OPC UA client rather than reporting a tested connection count [S1].
There is no universal OPC UA client limit: the answer depends on the PLC or runtime, operating system, address space, subscription count, polling rate, security mode, and available CPU and memory resources.
Published PLC and Server Limits
Siemens describes WinCC Unified as an OPC UA client for data acquisition in both Runtime and HMI form factors [S1]. The supplied material does not provide a tested number of OPC UA server connections for a WinCC Unified PC Runtime or Comfort Panel, so those figures must be confirmed with the vendor.
The Ignition OPC UA server has a hard-coded default limit of 100 active connections across Ignition 8.x, and the limit is not a licensing restriction [S2][S3]. An Ignition Gateway acts as one OPC UA client to the upstream server, regardless of how many Vision clients or Perspective sessions it serves internally [S2].
For a PLC acting as an OPC UA client, Siemens states a different resource limit: no more than 40 client instructions may be used simultaneously on S7-1500 and S7-1500T CPUs [S4]. This is the number of client instructions, not the number of external clients allowed to subscribe to the PLC's server.
Client Count Is Not Subscription Count
An OPC UA client is not the same thing as a tag. A Siemens forum example used 2 clients subscribing to about 160 items, including temperature and pressure sensors, on an S7-1500 OPC UA server [S7]. The example shows why a low client ceiling can coexist with a high item count.
Every active client can hold sessions, subscriptions, monitored items, and a publication interval. AVEVA states that each client connection increases server-side resource utilization [S5].
Use two separate checks for sizing. First, compare expected simultaneous clients with the vendor's stated connection or session ceiling, such as 100 [S2][S3], and confirm any Siemens figures with the vendor [S1]. Second, test the projected tag count, subscription rate, security configuration, and reconnect behavior on the intended industrial PC or PLC runtime.
Comparing Common Server Architectures
The supplied Siemens material discusses WinCC Unified as an OPC UA client but does not report tested connection counts for a PC Runtime or Comfort Panel [S1]. Ignition provides a default allowance of 100 sessions [S2][S3]. AVEVA Plant SCADA guidance describes fewer than 10 clients as the conservative recommendation, while stating that every client connection increases server-side resource utilization [S5].
Which Platform Fits the Use Case
A small HMI or panel-based architecture may suit WinCC Unified applications, but the applicable connection limit depends on the chosen runtime and must be confirmed with the vendor [S1]. An architecture with as many as 100 upstream connections may fit Ignition's default session allowance, but an external OPC UA gateway may still be needed when the application requires more than 100 [S2][S3].
For AVEVA Plant SCADA, use additional I/O Server or Connectivity Service instances when monitoring shows insufficient CPU or memory bandwidth [S5]. The industrial router or industrial PC layer can host a separate OPC UA server, but that changes network topology and adds another failure domain rather than raising the native PLC limit.
Large deployments should not treat 100 client sessions as proof of acceptable performance. Ignition's 100-client case involved clients monitoring only 10 to 20 tags each, while the intended deployment was 150 to 200 devices; the vendor explicitly left scaling performance uncertain and suggested testing [S2].
Security and Configuration Affect Capacity
OPC UA security modes change both configuration and processing load. Siemens lists 7 combinations across Security Policy and Message Security settings, including no security, Basic128Rsa15 with Sign or SignAndEncrypt, and Basic256 or Basic256Sha256 with Sign or SignAndEncrypt [S1].
Clients using signed or encrypted communication must also complete certificate handling. Siemens requires creation of a certificate authority, addition of the device, creation of the OPC UA client certificate, and installation of the server certificate [S1]. Certificate validation errors appear as refused connections, so they can be mistaken for capacity exhaustion during commissioning.
For a Siemens S7-1500 configured as an OPC UA client, the 40-client-instruction ceiling is a separate configuration limit [S4]. For the same controller acting as an OPC UA server, use the server's own documented limits rather than transferring the client-instruction figure to the server role.
Load Testing Before Production
Benchmark with production-like clients, tags, and publication intervals rather than counting applications in a design spreadsheet. AVEVA specifically recommends internal benchmarking, monitoring the Connectivity Server and I/O Servers, and refining the design before implementation [S5].
The test should open the planned number of sessions, subscribe to the complete item list, run for the intended polling and publication cycle, and reconnect clients simultaneously. Track CPU, memory, session acceptance, subscription behavior, and client-side data latency. This method distinguishes a fixed connection ceiling from a performance ceiling.
OPC UA aggregation provides another architectural option when data comes from multiple servers: an aggregating server can present a unified model so clients do not need separate connections to each source [S8]. That can reduce downstream client management, but it adds an aggregation layer whose own connection and throughput limits must be verified.
For an Ignition deployment, record the expected number of upstream OPC UA connections and confirm whether the 100-session default remains applicable to the installed 8.x release [S2][S3]. For Siemens, verify whether the design is a WinCC Unified server, an S7-1500 server, or an S7-1500 client. The supplied material states a 40-client-instruction limit for an S7-1500 or S7-1500T acting as an OPC UA client [S4], while WinCC Unified connection figures must be confirmed with the vendor [S1].
The verifiable procurement action is to obtain the exact server model, runtime version, expected simultaneous client count, subscribed item count, sampling interval, and security mode from the OEM, then confirm those figures against a production-like load test.