It must be clear, however, that not every timing program allows input from every arbitrary device. There may be programs that definitely require a certain device in order to work properly. Suppose you want to measure response times. Using the keyboard as a response device will be a bad idea in this case, since there will be a considerable delay between the subject's response and the software interrupt that is needed to detect it. Thus a program UTC to measure response time may require a switchboard. In this case the program will check the current value of switchtype before it starts. If the switchtype value is not appropriate, the program will issue a fatal error message and stop execution.
Each aviation computer timing board has a keyboard that may be used as a response device. For many experiments a keyboard is sufficient or even necessary.
One major problem with keyboard input is that the system's keyboard software usually is not very useful for running experiments. PXL provides some but not too much help for MS-DOS users. PXL programs intercept the keyboard interrupt service in order to get the interrupt time as early as possible and they provide a real BREAK key, that even works if the program runs in a dead loop not doing any system call.
Thus a PXL program may be stopped any time by pressing the CTRL-C key combination. Such a break closes all open files and restores the original system status. The break mechanism works on MS-DOS systems but only as long as the keyboard buffer is not filled by other nonsense input.
Timing switchboards on IBM PC/AT compatibles may be connected to the parallel printer port or the game port. The printer port provides 5 handshake input lines that may be used to connect up to 5 digital switches to the computer. Figure prswitch shows a simple passive network to connect 5 switches to the IBM PC parallel input connector. Figure gswitch shows how to connect 4 digital switches to the game port connector.
Note from Table scode that the timing switchtype codes for switches do not stand for the number of switches but for the configuration. Thus number 4 is a switchboard with four switches in a single row, while number 5 stand for 4 switches in a square. Figure sboard shows the suggested configurations for switch boards and the corresponding numbering schema.