Abstract: A signal processing device includes a proprietary test mode symbol generating circuit, a decision error detection circuit, and a debug control circuit. The proprietary test mode symbol generating circuit generates prediction symbols as a reference signal according to decision symbols output by a slicer of a receiving signal processing circuit and a predetermined rule. The decision error detection circuit operating in a proprietary test mode continues receiving the decision symbols and the prediction symbols, and generates detection results. The debug control circuit includes a memory device, and continues recording contents of one or more signals obtained from one or more nodes of the receiving signal processing circuit into the memory device. In addition, the debug control circuit receives the detection results, and stops recording the contents of the one or more signals in response to a state of at least one of the detection results.