IBM 5161 Expansion Unit – The Components

During my time as a member of the Vintage Computer Forums, and of course, after learning about the existence of the IBM 5161 expansion chassis, or unit whichever you prefer, I have been fortunate enough to have made numerous friends. Most of the friends I’ve met have some knowledge of the expansion unit, but there are those who have no clue.  I have to admit, learning what I have in regard to the expansion chassis took some time.  Most of it, before actually acquiring my own expansion unit.

It was during the time I was researching the 6161 for myself that I learned about what the expansion unit was, what its intended purpose, and most of all, what makes the expansion unit what it is. All-in-all, the expansion unit isn’t all that complicated.  And, while I won’t be going into any of the most technical areas in terms of what makes certain components work, I will provide a brief description as to what each component is, and its place in a working IBM 5150 PC, or IBM 5160 XT installation.

To begin, let me introduce the components that make up the IBM 5161 expansion unit, model 1. (You will learn the differences between the model 1 and model 2 as you continue reading)

  • Case
  • Power supply
  • Planar or Back Plane
  • Internal hard disc drive
  • Hard disc controller
  • Extender card
  • Receiver card
  • Connecting cable


As you can see here, there aren’t a lot of items that make up an expansion unit. There are however a couple of key differences between a Model 1, and Model 2 units; most notably, the inclusion of an internal hard disc drive and controller.   Model 1 expansion units were manufactured with the 5150 PC in mind.  Since the PC initially had no provision for the inclusion of an internal hard disc, and the PC also typically did not have a strong enough power supply, the expansion units, models 1 included a 130 watt XT power supply unit, or PSU.

However, here is where the primary differences between the different revisions come into play.

IBM 5150 PCs included only five (5) ISA expansion slots during their manufacture. In fact, no REAL IBM 5150 PC had more than five.  So an expansion unit built for the 5150 had to include at least one internal fixed disc.  One full height, 10 Mb MiniScribe hard disc was installed, along with a Xebec fixed disc controller.  One additional fixed, or hard disc drive could be added to the expansion unit for added flexibility by adding the additional storage space.  With two internal hard discs, storage capacity could be doubled, tripled, or by adding larger capacity hard discs, multiplied to include as much as much storage as the use desired or afford.  Keep in mind, that during the time of their construction, 10 Mb hard discs cost nearly as much as a new car!

IBM 5160 PX/XT systems were by design, built to include one internal floppy disc drive, and one fixed, or hard disc. If an XT user were to purchase an expansion unit for their XT system, the expansion unit they would purchase would, first of all be less expensive because it did not require that a fixed disc or controller be fitted, or included.  Indeed, it was recommended that the hard disc drive and controller were to be removed from the 5160 XT and compatible systems and moved over to the expansion unit.  This saved the consumer some money at the time of purchase.  However, the user could still add an additional hard disc for expanded storage.

Regardless of which expansion unit was purchased, Model 1 and Model 2 expansion units had eight expansion slots. They both used the very same case, planar, power supply extender and receiver cards.  The only real difference was whether or not an internal fixed disc, and appropriate controller, were included.

Let me begin now by looking at the individual components that make up the 5161 expansion unit.


  • The case


The case looks almost exactly the same as any other IBM 5150, or 5160 computers. IBM chose to keep the same case design for simplicity purposes, as many of the same components that make up a 5150 PC, or 5160 XT and the expansion unit are the same.  Therefore it made sense to stay with the same case design.  Plus, by staying with the already proven design scheme, there was a particular continuity when assembling an IBM 5150+6161 or 5160+5161 system.  By having both the computer and expansion unit look the same, it was easier to maintain a single inventory of let’s say, the metal case upper and lower; the plastic bezel, etc.  In reality, the only things that had to be unique for the expansion unit were the labels, and badges that indicate which unit it was initially designed.  Take a look at a 5150 PC for instance.  The front bezel is the same as the XT. The same goes for the power supply, and case materials.  The only differences were in the case badges.  For the 5150, the badge reads, “Personal Computer”, where the XT’s reads, “Personal Computer XT.” The badge on the front of the expansion unit reads, “Personal Computer Expansion Unit” regardless whether it was a Model 1 or Model 2.

  • Power Supply


The next item on the list of components is the power supply, or PSU. In 5150 systems, the power supply was limited to only 63 watts!  In order for the 5150 PC to have enough energy to power the computer and a hard disc drive, the PSU needed to be rated at 130 watts or greater.  Any user who opted to add in a fixed disc to their 5150 systems had to also upgrade the power supply in order to have enough energy for the system to run.

However, for the 5161 expansion unit, IBM chose to use a 130 watt IBM PC/XT power supply. This was necessary to power the expansion unit and its internal ISA expansion cards, and the use of up to two internal hard disc drives.

  • Planar or Back Plane


The planar used by the expansion unit is another one of the “off-the-shelf parts IBM chose to use in constructing their PC line of computers and is in no way a proprietary unit. The primary layout is the same as a standard system board, but without any of the logic circuitry required by a full-fledged computer mother board.  Instead, there is a standard 12 volt power connector, 8 ISA expansion slots, and little else.  Remember, early PC’s used ISA slots for any add-on associated to the computers construction and operability.  In a standard IBM 5150, two (2) of the computer’s five (5) 8-bit ISA expansion slots were already used.   That left little else in the way of upgradeability.  In addition to the computers floppy controller, and video controller things like a memory expansion, parallel and serial ports used all of the computers available expansion slots.  So adding a hard disc, or game controller were relegated to the expansion unit.  Some users worked around these limitations by buying combination expansion cards; like the AST Sixpack Plus which included, a clock, calendar, parallel port, asynchronous serial port, Game controller port, and a memory expansion.  These expansion cards went a long way in freeing up resources in the 5150 PC, for adding in a hard disc controller, but for those who needed even more expandability, the expansion unit was really the only option.  Having eight (8) open ISA slots to add items such as a  modem, or LAN adapter, in addition to additional serial ports, meant the difference between having a computer one could work with, or having to work with multiple computers to do the same job.

  • Internal hard disc drive
  • Hard disc controller


As mentioned previously, purchasing a hard disc drive during the early 1980’s was a very expensive proposition. Buying two hard discs was almost unheard of in the private sector.  That meant that almost all of the IBM 5161 expansion units were purchased by small businesses, and adding an additional fixed disc or in number was done by larger corporations and those businesses who absolutely needed the additional storage capacity.

Still, the requirement for at least one hard disc is the primary reason the expansion unit was developed by IBM in the first place. Colleges, and the educational sector accounted for the purchase of a very, very small percentage of expansion units, as operations, and educational usefulness could be handled by the 5150, or even 5160 XT computers, outfitted with dual floppy discs.

Hard disc drives, being as expensive as they were, were also used in locations where more permanent storage of records was needed. It is that reasoning that one hard disc was included in those expansion units which were destined to be connected to the 5150 series of computers.  Since IBM began including a fixed disc in their XT line of computers, including one in the expansion unit was seen as redundant.  However, by allowing the expansion unit to use more than one hard disc, and one with more storage capacity than the 10 Mb MiniScribe drive they initially chose, they allowed the computer user to add as much storage as they needed, or their pocketbook would allow.

By allowing, or including one, or more hard disc drives was important, none could operate without the prerequisite controller card. Xebec, Western Digital, and others soon made appearances in expansion units as newer larger hard disc drives were added.  The Xebec controller was just not sufficient at controlling more than a small handful of drives.  Manufacturers of hard discs would often recommend other brands of controllers which were compatible with their drives, or included a controller that would drive not only their own hard disc, but earlier drives as well.


  • Extender card
  • Receiver card
  • Connecting cable



Both cards, and the connecting cable are required in order for the expansion unit to function with the PC. Regardless how many of the other components are available and assembled, if the extender; which is installed into the PC, the receiver; which is installed into the expansion unit; or the cable to connect the two.  The cable is described as follows:  a 56-wire, foil-shielded cable terminated on each end with a 62-pin D-shell male connector.

One of the most commonly missed components of the IBM 5161 expansion unit is in fact, the extender card. One reason for this is that while many complete systems are placed in storage, it is often only the PC that is either given away, or sold.  Most often, the extender card is found in the PC, and discarded.  We now have an orphaned expansion unit.  Then when the expansion unit is later found, it is missing the extender card, and cable.  But while the cable can be replaced, the extender cannot.  If an extender card is found, it must be an authentic IBM part, or it will not work.

Currently, there are less than 20 IBM 5161 expansion units in complete working order! That is important!  That is – Less than 20 expansion units in complete working order!!! The primary reason for the possibly many others to not fall into that category is simply because of the missing extender card.

All other reasons for expansion unit failures are insignificant in number. It must be mentioned though that if one were to possess an extender, receiver card, and cable, using this list of parts, one could theoretically build their own expansion unit.  Most of the parts are freely available online.  Specifically, the power supply, 8-bit 8 slot passive ISA backplane etc.  Once these components have been assembled into an IBM PC/XT computer case, it could work.


I have seen one, and only one of these home built expansion units work. It is NOT being counted as one of the original IBM expansion units, because the only items from an authentic IBM parts list are the extender, receiver, and cable.

If there are any questions regarding the assembly of an authentic IBM 5161 expansion unit, or in its operation, please feel free to contact me. I do NOT however, have access to spare parts, but I can assist in completing your own build once you have a workable unit.

Thank you for reading.





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