Request Scheduled Keyword
MDSP bit 100 modifier, the bit that controls whether the next item's display is scheduled a constant time from the request. Reset if N = 0, otherwise set. All MDSP modifiers are both parameters and switches. If N is missing the bit is set and the mode is active.
Given that the <Delay> keyword's use does the same thing as this mode and that almost every use of this mode is going to involve specifying a delay this mode is superfluous.
If a polled input device (see Input below) is being used and you are not using continuous running then you may want to reduce the default polling rate from 10ms otherwise your ISI will vary by up to 10ms.
By default (no <RequestScheduled>) DMDX will take as long as it needs and then add 48 ticks to that for an ISI, if you set <RequestScheduled> then everything DMDX does in between items must occur in under 48 ticks, if 48 ticks is not enough time you are responsible for determining the correct amount of time that DMDX will need to complete it's houskeeping (usually from error messages that indicate the size of the error they encountered) and using that in a <Delay> parameter (thus making <RequestScheduled> redundant).