Chose the time-base clock. It defines how long 1 time-unit will be. The choices are:

20MHz (1 unit = 50 ns, default)

1MHz (1 unit = 1 μs)

software-controlled (generated by the onboard timer, 1 unit = N ms)

external clock source (connected to X-CLK input on the front-panel of the Control Module; use TTL-compatible square-wave generator of good quality; do not exceed 0…+5 V range).