¹⁵Some systems such as the Texas Instruments 505 series used “X” labels to indicate discrete input channels and “Y” labels to indicate discrete output channels (e.g. input X9 and output Y14). This same labeling convention is still used by Koyo in its DirectLogic and “CLICK” PLC models. Siemens continues a similar tradition of I/O addressing by using the letter “I” to indicate discrete inputs and the letter “Q” to indicate discrete outputs (e.g. input channel I0.5 and output Q4.1).