NOTE: Concerns about formatter reconfigurations will disappear when the digital backends take over. About half of VLBA projects use the new system as of the start of 2013 and that should go to all during the year, so soon this section will be obsolete.
A factor to consider when planning scan times is legacy formatter reconfigurations. These happen when the internal switching and setup of the formatter at the station has to be changed. Such changes happen when any of a number of parameters, including the number of channels, the sample rate, the BBC assignments, the BBC sidebands, and the pulse cal detector setup, are modified. A formatter reconfiguration takes about 8 seconds on the VLBA and during that time the formatter is not sending valid time codes to the recorder. If this happens during recording, it knocks the correlator out of sync for both the duration of the reconfigure plus the time to resync. In practice, it also seemed to confuse the old VLBA hardware correlator somehow for maybe one or two stations and they can take over a minute to resync. This is not likely to be the case for the software correlator.
It is best to avoid reconfigurations if possible. In the rare cases where that is not possible, try to provide a gap between scans of sufficient length to do the reconfiguration. The formatter configuration requested during a gap is the same as that during the following scan, so this only requires using the GAP command (or any other mechanism for having one scan start a while after another ends). Common reasons that reconfigurations occur in schedules are changes in the sample rate, changes in the BBC sideband (remember for net upper sideband, the 20cm and 13cm systems on the VLBA use lower sideband at the BBC), and changes in the kHz part of the frequency which changes the pcal detector frequency (changes in the MHz part will not change the pcal setup and will not trigger a reconfigure).