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In most cases, the server can support as many client connections as needed; however, the number of process threads for a running application process is limited.

• Windows XP and older operating systems limit the number of threads per process to 2024.
• Windows 2003 and higher operating systems limit the number of threads per process based on the resources that are available.

In OPC, each client connection consumes a single thread; each OPC group that is added by an OPC client connection consumes two threads in the server. One additional thread is usually needed per channel for device communications and other purposes. As a result, the maximum number of connections can vary from project to project. In some cases, a project that is moved from an older XP operating system to a new operating system may support fewer connections than expected.

Note: When the thread resource limit is met, attempts to add groups and items will fail with error code 0x8007000E ("E_OutofMemory").