I'm having a hard time understanding a historical use for these as well. If I had one I wouldn't know what I'd use it for. Does anybody know what these expansion units were sold for?
Like Shadow Lod stated. There were initially sold as a way to add hard drive functionality to the 5150. The added expansion slots were nice too, but I'm like you. Why would IBM make such a thing when you couldn't completely fill both units? There are only so many IRQs, and those are used up pretty quickly.
What were they initially sold for? Well, ole, I thought you had one of these and the extender and receiver cards you gave me were extras you somehow collected. But to answer your question, I really wish I knew. The closest thing I can come up with is the product release notes from IBM, and even they don't give any kind of reason for making it available. So we're all left with the same explanation of being able to add a hard drive to the 5150 due to the enemic power supply of the original PC. IBM could have just as easilly made a hard drive installation kit that included a more robust power supply. And it would have cost users a lot less.
And like Shadow Lord expresses, it is like having a fully functional XT, although one with dual floppy and hard disk drives, plus more expansion slots. If it werent for the fact that I do have a multi card, the AST Six Pack Plus, I would need the extra slots to add all of the functioanlity. So, the expansion unit did have it's place. To be honest, I have the exact oposite problem as most though. I'm surprised that IBM sold as many as they did for the shher fact that multi cards were available, and the need for more expansion slote diminished dramatically.
My own reason for wanting one stemmed from my desire to not only build the best possible example of a complete IBM 5150 system, but to preserve some of the lesser known peripherals for the future. If I had not run into that IBM5150.net, and learned of the existance of the IBM 5161 expansion unit, I would have gone on blissfully and not caring that it was even out there. But because I obsess over everything; Well, of course, I had to gwet one. Now that I have mine, and it's working with my 5150, with the exception of a problem I'm having getting my hard drive low level formatted, (See my new thread here:
http://www.vintage-computer.com/vcforum/showthread.php?31243-Debug-and-Low-Level-Format) I love the way it works. And the fact that certain things have to be done in a certain order, make it all the more interesting to me; like the simple act of booting the system for example.
