The PPG System is not a specific product in it's own right, but more like a mix of several products always consisting of at least a PPG Waveterm A/B and one or more PPG Wave 2.2/2.3/EVU and e.g. a PPG PRK/PRK-FD.
The seperate components had to have their own ID number (kind of like SCSI ID's) to make sure the Waveterm could send and receive data correctly to the corresponding unit.
This meant that if you wanted several PPG Wave 2.3's set up in a system for instance you had to have a special EPROM made that would take care of this ID number as it was not possible to change in the operating system. Also several PPG EVU's would require the special EPROM with a different unit ID burned into it.
A maximum of three sound producing devices could be included (for instance a PPG Wave 2.3 and two PPG EVU's), a PRK FD and a PPG HDU.
The PPG System even in it's smallest configuration was NOT a cheap system by any means, but on the other hand it was also a very VERY powerful system that would provide musicians, producers and studio techs with enough power to create complete hits (if you had the creative skills of course :-).
Some tweaking/cabling and DIP switch switching have to be made in order to make the system run smoothly, but once it's done you have a complete, incredible sounding and professional system that can do some stuff that's hard to do even on todays hardware.
The PPG HDU could also be added in a PPG System, and - as far as I know - could be used to save samples on the internal HDD instead of the PPG Waveterms internal floppy drive.
For more info on the seperate components in a system click the instruments or computers in the menu.
Read more on the PPG System in the official prospect which you'll find in the Documentation section or right here.