User logic within a PLD is debugged by way of the hub. The PLD includes a serial interface (such as a JTAG port) that communicates with a host computer. Any number of client modules are within the PLD and provide instrumentation for the PLD. A module is a logic analyzer, fault injector, system debugger, etc. Each client module has connections with the user logic that allows the instrumentation to work with the user logic. The hub communicates with each client module over a hub/node signal interface and communicates with the serial interface over a user signal interface. The hub routes instructions and data from the host computer to a client module (and vice-versa) via the serial interface and uses a selection identifier to uniquely identify a module. The hub functions as a multiplexor, allowing any number of client modules to communicate externally though the serial interface as if each node were the only node interacting with user logic.