The SX/12 Data Channel Simulator creates a controlled environment for testing, tuning, or demonstrating data communications equipment. Its delay buffer and error generator simulate the major characteristics of satellite and terrestrial data channels. Delays and/or errors can be selected to assist in hardware and software performance testing, stress testing, optimization, and off-line development. Programs can be entered to simulate complex error scenarios or test sequences. The SX/12 can also be added to real data channels to insert additional delays or errors to test their effects on the on-line communications equipment.