In addition to their mandatory carriage on SOLAS vessels as mentioned, some flag states also require that VDRs be carried on certain other vessel types that trade only domestically and are therefore outside the scope of SOLAS. A VDR is not a single piece of equipment as the name suggest but is a system for collecting and recording data that involves electronic feeds from navigating equipment and machinery systems on board, sound and video recording devices that capture human involvement and reaction, a central control cabinet and data acquisition unit and a data recording capsule designed to protect the data in the event of any manner of disastrous event including fire, explosion and sinking. In some instances a float free data capsule is part of the system. To be compliant with the regulations covering their carriage, VDRs need to be connected to the navigating instruments, alarm systems and the majority of controls on the bridge. In addition they must be able to make audio recordings of the bridge environment so that conversations and orders occurring before and during an incident can be accessed as part of any official investigation. Because of the number of connections that need to be made, fitting a VDR is not a straightforward task and cost of the equipment and fitting can be considerable. When first introduced, the rules required the recording media of a VDR to be capable of recording at least 12 hours of continuous data after which it could be overwritten. The majority of VDRs were able to record for much longer than the minimum allowed but there are now much more stringent requirements in place. Recording media has improved greatly since VDRs were first introduced with hard disks now often replaced by solid state memory which is much more robust and less prone to damage. When originally mooted as an item of mandatory equipment around the turn of the century, VDRs were initially considered as an unnecessary surveillance of crew activity but have since become an accepted part of bridge equipment. There was too, some criticism about the data recording capsule not being required to be a float-free device; the argument being that unlike aircraft black boxes, VDRs will almost certainly end up at the bottom of the ocean in worst case scenarios. Another point of concern was that given the range of equipment that would need to be connected to a VDR, it might not be possible for existing ships to comply with the rules contained in SOLAS and the accompanying performance standards. This was recognised as a valid point by the IMO and eventually a simplified or S-VDR standard was formulated for vessels unable to comply with the full version. It was further permitted for flag administrations to dispense with even the S-VDR and exempt ships, other than ro–ro passenger ships, constructed before 1 July 2002 from being fitted with a VDR where it could be demonstrated that interfacing one with the existing equipment on the ship was unreasonable and impracticable. The original performance standards for VDRs are to be found in IMO Resolution A.861(20) from 1997 and those for S-VDRs in MSC 163(78) adopted in May 2004. The latter S-VDR standards introduced the possibility of float free capsules leading to the development of S-VDR SARTs by some manufacturers. IMO Resolution MSC.214(81) adopted two years after the S-VDR standards were defined, introduced a requirement for data download capability on both VDRs and S-VDRs. In May 2012 the standards for VDRs were further refined by Resolution MSC.333(90) which added a requirement for data from more equipment including ECDIS and Inclinometers if fitted for any VDR installed after 1 July 2014. The new requirements have been a catalyst for some makers to withdraw from the market mainly because the anticipated volume of sales for new models is now quite small and because the necessary changes to equipment would make production uneconomic. The withdrawal of manufacturers does not make existing equipment invalid or non-compliant but lack of support may mean that a new compliant replacement will be necessary in the event of failure. The new standards also make a float-free recording device compulsory and have increased the minimum recording times requirements. The current performance standards including the 2012 revisions are now covered by the annex to Resolution A.861(20).