Normally, SCHED eliminates stations for which the source is not up if they are using a disk recording system. DODOWN with no argument overrides that behavior and keeps stations in scans regardless. DODOWN can be set differently for each scan. Setting it to any non-zero value sets it to false (ie - go ahead and remove stations).
In the tape era, only VLBA stations were eliminated and only if AUTOTAPE was set to 2.
Without DODOWN, SCHED will also take a station out of a scan if the predicted slew time is such that the antenna will not reach the source before the scan end. However, it was found that this can prevent wraps from occuring in a timely manner during fast switching operations such as phase referencing, when one source needs the wrap a while before the other. Therefore, SCHED will not remove a station from a scan if mount type is ALTAZ and the requested azimuth slew is greater then 315 degrees. Such long azimuth slews would normally mean that a wrap is needed so getting it started is useful. There is no perfect algorithm to always do the right thing here so some though may be required by the PI if the behavior is odd. DODOWN is now scan dependent to help with dealing with odd behavior. Also explicitly including or excluding stations from the scan can be a strong tool for dealing with problems. Beware of special instructions when there is a chance that a program will be time shifted for dynamic scheduling. That will change when things like wraps need to happen.
Note that SCHED does not presume to tell the antenna what wrap to be on, mainly because there is no mechanism to do so with the VLBA. Some day this may change in which case SCHED can be more intelligent about what to do. For now all SCHED can do is control what scans are present and guess what the antenna will do.