Serializer, deserializer, and/or serdes ICs are configured to support one or more test modes to enable end-to-end testing in communication links in which the ICs are implemented. To support the end-to-end testing, the ICs can include a multiplexing stage with means for deterministically mapping a plurality of input parallel data signals to at least one output serial data signal and/or a demultiplexing stage with means for deterministically mapping at least one input serial data signal to a plurality of output parallel data signals. When used in combination in a communication link, the means included in the multiplexing stage and demultiplexing stage deterministically map specific input parallel data signals to specific output parallel data signals.