SCHED was originally written in about 1979 as the scheduling program for global VLBI observations using the Mark II recording system. That was a system capable of recording 4 Mbps on video tape recorders. It was in use in some parts of the world much longer than expected (at least to 2003), but is now gone as far as we know. SCHED version 9.4 and later does not support Mark II.
The wide band recording system in use for VLBI for many years was the Mark III system. SCHED was never a general purpose scheduling program for Mark III observations. Programs SKED and PC-SCHED served that role. However, SCHED was capable of scheduling Mark III observations on systems that used the VLBA control files (VLBA, VLA, and Green Bank). In fact, the output of SKED and PC-SCHED was normally processed through SCHED to produce the telescope control files for these antennas for all except geodetic projects. Mark III is also obsolete, although one Mark III schedule is still in the SCHED test suite. I am not aware of any Mark III systems remaining in use.
The Mark III system was replaced by several other systems with greater capabilities. These include the VLBA, VLBA4, Mark IV, S2, and K4 tape systems. There were correlators associated with each system, and there was a considerable amount of cross compatibility, either directly or through the use of translation machines. The Mark III, VLBA, VLBA4, and Mark IV systems all used the same tape transport, although with different electronics. In their native modes, they used different data formats, but the VLBA and Mark IV systems were capable of reading and writing Mark III data. More importantly, the VLBA and Mark IV systems have a wide range of compatible recording modes that could be correlated together on the VLBA, JIVE, and other correlators.
By 2007, the tape systems were replaced by disk based recording systems nearly everywhere. The early disk systems were plug compatible replacements for the tape drives so most of the data configuration remained unchanged.
The Mark5A system was the most widespread early disk system. It was developed by Haystack Observatory and the Conduant Corporation. As a plug replacement for the tape drives, the Mark5A system still has the concept of tracks.
The Mark5B system is an enhancement that uses the VSI standard interface to record the data channels without all the formatting baggage left over from the tape systems. The Mark5A+ system allows Mark5B recordings to be played on Mark5A playback units, but a minor glitch that nobody is in a position to fix is preventing its use on the VLBA. The Mark5B+ system uses a faster interface and can handle up to 2 Gbps.
The Mark5C system was being deployed in 2011 and is meant to record up to 4 Gbps, although it the initial configuration can be used at a maximum of 2 Gbps. The Mark5C- system is a scheme for allowing lesser Mark5 hardware to pretend to be a Mark5C system for system development.
As of early 2013, the VLBA winding down use of the Mark5A system and is moving most projects over to the Mark5C system. Some other observatories are also moving to Mark5C, but usually with different electronics.
There are other recording systems in use. The Japanese have a K5 system. The Australians are using a variant on the PCEVN system. Also a variety of groups are testing real time VLBI over the fiber networks. This can involve real time correlation, or recording at sites remote from the telescope.
Meanwhile, other potential future recording systems are being investigated.