IBM 5120 (aka IBM 5110-3)

(work in progress)


The IBM 5120 is also known as the IBM 5110-3. In the system I received, some of the manuals say “5120” while the parts summary in the sales paperwork states “5110-3.”

I’ve verified that the A1 board cards between the two systems are fully compatible. Or at least, I’ve verified: the RWS (RAM), Executable ROS, Disk Sort, PALM CPU and IO card, Display card, and BASIC ROS.

Manuals Galore

Many physical/printed manuals were included with this 5120. That’s often a good sign of having found an original owner of the system (and higher chance of getting original disk or tape media also).

Of particular note, I found two IBM “notice to customer” bulletins that included a catalog of software titles from IBM and the corresponding price (either by a monthly license or a “one-time-cost” charge).

I’ve placed a photo of each page of those notices here. While these bulletins are 1985, a lot of the software titles do apply to earlier and other IBM systems (5100 on up to S/36).

Here is a sample from those bulletins:

So Many Issues

The 5120 I received had several issues.

Issue #1: The Fuse Issue

At first the system wouldn’t display anything at all (the fans would spin up). An issue with the fuse didn’t occur to me initially. What I did first was pull the A1 board cards. As with both the 5100 and 5110, IBM used some kind of “foam board” to help keep the system cards in place. But the material they used has ended up deteriorating in a messy way with age. However, on the 5100/5110, the A1 board “flips over” so this foam board ends up below the cards. The 5120 A1 cabinet doesn’t have this “flip” mechanism and so the foam board ends up above the cards! So all the deteriorating debris was “raining down” on the cards.

I removed all the cards and removed all the largest chunks of the foam board, then vacuumed out what I could on the smaller debris parts. Some of the foam board had baked into a “goo” or “thick tar” like substance that was harder to remove. Fortunately this was only along the portions of the slots that are not used (only because this system configuration had no async or comm. cards). I left the “sticky parts” for now, since I was concerned more aggressively removing it might damage the pins there.

Here is what the “cleaned up” A1 board compartment area looks like. Note that this compartment slides out from the back of the 5120 (after removing the upper and lower parts of the case), whereas on the 5100 and 5110 it “folds out.”

I re-seated all the cards in the cleaned up bay, but the system still didn’t startup properly (nothing on the display). But as I was re-seating the cards, two things occurred to me. In the 5110 MIM I recalled reading about the system fuse that was below the CRT display – I had to replace one of these in one of my 5110 systems. Then in the 5120, I noticed a very obvious fuse enclosure right behind the CRT itself — evidentially IBM had relocated this fuse to this easier-to-access location.

The fuse was not only blown (no wire), the end caps of the fuse also just slid right off the glass. Maybe it was micro-vibrations during transport. In any case, it’s the same fuse as used in the 5100 and 5110, so I had spares on hand. I replaced this fuse and content now started to appear on the display during startup of the system!

Issue #2: Bad Executable ROS

The Executable ROS appears to be corrupted. This is unfortunate since I was hoping to replicate the “ROS Extraction” procedure that Corti and Stepleton had done on the 5110 and 5100, to see if this ROS was any different than the prior systems. Mainly you can tell when the Executable ROS is corrupted, or some aspect of startup is “not right,” by going into STEP MODE and Display Registers. During the very first cycle of startup, the top left should have “000A” since that is the very first executable instruction address hardwired into the system. If it has anything else, then the system isn’t starting at the proper address and will then have an unpredictable behavior. In this case, here is what the startup screen looked like.

To further verify there was an issue with this 5120 Executable ROS, I swapped this card into one of my IBM 5110 systems. With no other change to the 5110 system, then on startup of the 5110 it showed the same issue: “garbage” on the Display and no normal startup sequence. This indicated that there was indeed a card issue, not some other power or connector issue.

I then placed the Executable ROS of the 5110 into the 5120 system. Before doing so, I did two things: The 5120 I received happened to have an un-opened copy of the 5120-specific SLM, which to my knowledge hasn’t yet been digitized. I have digitized the 5110-SLM, so I compared the pinouts of the Executable ROS between the two systems to verify all the same pins are being used. Everything looked identical as far as I could tell. The second thing I did was visually compare the two cards, which is shown below.

I can tell the upper-card in the photo above is from the 5120 because of the shape of the jumpers on the right side of the card (and because I know the one from the 5120 has the white “MB” circular sticker on one of the chips). I swapped in the Executable ROS from the 5110 into the 5120, and the “garbage display” on startup went away! The system would now try to boot.

Issue #3: Bad Common/Language ROS

The next issue was during the startup CRC checks, apparently there was an issue with ROS “40.” From my work with the IBM 5110, I know ROS 40 is related to the Common ROS (which has ID’s 40 and 41). The Executable ROS itself is not CRC checked, and it contains the code to perform the CRC check of each ROS module (that is at the end of each 6KB segment). The image below is what this next problem looks like:

NOTE: The above image is using the inverted WHITE background, to show that this feature does still work on this system. On the 5100/5110, this was a switch at the front panel. On the 5120, this is a switch inside the system near the RUN/STEP controls. The option is still present on the 5120, just not presented as a front panel switch.

The Common/Language ROS contains “common code” and handle things like converting keyboard scan codes into the code used by the active language. This ROS card also holds the BASIC language itself, as implemented in System/3 instructions, and the “BASIC ROS” emulation code implemented in PALM. So, I knew this was a bad sign and not an easy thing to fix.

I’m not sure what the rest of the errors are: “ID 03 ERR” and “ERROR 001 D80.” I tried entering the DCP to see if I could just by-pass this error (by jumping to the next part of the startup sequence in the Executable ROS), but no luck in doing so (I could enter the DCP, but could not type anything to invoke the “BX” command). The “ERROR 001 D80” is related to the disk drive system and separate from this startup issue.

The only workaround here was to also borrow a working Common/Language ROS card from the IBM 5110 (again after comparing the pinouts in the SLM documentation to verify they are the same).

This indeed did get the system booting up to a BASIC prompt! But it has to borrow two cards from the IBM 5110 to do so. So my “net gain” of working cards is a 32KB RWS (RAM) cards, the PALM processor, and the Display card. (and a Disk Sort and External IO card)

Issue #4: Process Check [not yet resolved]

While the system is processing BASIC commands, the “Process Check” light at the front panel is still remaining illuminated. This indicates *something* still isn’t quite right in the system. When I replaced the fuse behind the CRT, I recall also noticing a ground wire at the back of the system that seemed slightly loose. I did tighten that ground wire back up, but perhaps there are similar issues like this deeper inside the system.

NOTE: The MIM/MAP documentation contains lots of notes about diagnosing the “Process Check” light, but it’s a little confusing since it depends on a CE tool probe that has an “UP” light. It’s not exactly clear on what voltage conditions cause the “UP” to illuminate if one had that probe.

Tentatively, my best guess is that the remaining black residue on the B/C/D cards might be causing this “Process Check” light. As I’m not using any of the cards in those slots, hopefully this means there is no real issue here.

Issue #5: 8″ Disk Drives [ not yet resolved ]

In all the documentation that did come with the system, only one 8″ IBM Diagnostics disks was included. So far I still can’t get a directory listing of that disk.

It occurred to me that the system came with a printer – and that printer has a built in “terminator.” Even on the IBM 5100, the external tape system (IBM 5106) also needed a terminator. This basically marks the “end” of the external peripherals. Perhaps this 5120 was always used with the printer attached? On that note, why not just borrow the terminator from the IBM 5106? I wasn’t sure if the built-in disk drive unit of the 5120 needed a terminator or not – but I figured it didn’t hurt to try.

But the first problem was I could not fit the printer attachment nor the terminator borrowed from the 5106! The cover from the Asynchronous/Comm ports was in the way, I could not fit the attachment into the External IO pins. This didn’t seem right to me, so I thought about it. Eventually I realized that even with the case closed up, the cover for the Asynch/Comm ports could slide over a bit! That opened up space so that I could now attach either the printer or terminator (as shown below).

After doing this, I then would consistently get “ERROR 045 D80” during startup (i.e. with the terminator connected). What’s interesting about that is ERROR 045 means “disk not inserted” which is accurate! I hadn’t inserted and closed the drive door. It seems that when a terminator is connected, a disk is required during startup of the system.

So I did insert the only 8″ disk that I had available and closed the door. The system would “boot” to BASIC, but issuing a “UTIL” (directory) still then only give me ERROR 001 (a kind of general disk read error). I sometimes also get ERROR 033. So at this point, the disk drive system isn’t working (or the only disk that I have is invalid).

A brief video of the disk drive while “uncaged” and doing some bench testing is here.

I also checked the fuses associated with the disk drive system, they seemed fine. Per the manual, the keyboard has to be removed to accesses these fuses – and indeed that is the case, they aren’t really on a pull out base and are best accessed behind the keyboard.

NOTE: To avoid damage to the original IBM Diagnostics 8″ disk, I ordered a set of 3M DS/DD (or “2D”) disks to use while I’m troubleshooting. However, apparently to use them they have to be “low level formatted” using a program called INITIAL. That program is on another IBM disk not included with this 5120, it is called Customer Support Functions. After a disk is initialized, only then does the MARK command work. Corti’s original emulator tar package includes this CSF.DSK which contains the INITIAL program), but at the moment I’m not sure how to encode those images back to media.

Issue #6: The 5103 Printer

I’m not terribly interested in vintage printers. They’re neat, but the ink, cartridges, and paper can be hard to find – plus they’re just noisy (but the screeching noise is also part of their “charm”). But this system came as a set, so I had to take it in.

I did try to connect the 5103 printer – it has this “dangling cable” that has a bulky “tri-connector” (with two 24-pin and a single 15-pin connector) that attaches to the back of the 5120. Actually, this 5103 should be compatible with the 5100 and 5110 as well – and in fact, the 5110 is what I really tried it on first.

So far no response on the printer. Even actually using the printer (via software) was a little obscure. I think from BASIC, a sample command would be:

10 PRINT FLP          <-- blank line

But so far I just get ERR 13 500, which I think is indicating the device is not attached (even though the cable seems secured). The IBM 5110 MIM refers to a specific 5103 printer manual, but aparently that manual isn’t on bitsavers yet! So I’ll dig through the set of manuals that came with the 5120 and see if it is in there.

Additional Photos

GROUP 1: Miscellaneous (Inverse Switch, RUN model, STEP switch)

GROUP 2: CRT internals profile (speaker at rear of CRT)

NOTE: There is actually a 2nd speaker near the up-front controls (Register and Reset switch). On the earlier 5110, the speaker was only present behind the CRT.

GROUP 3: Folding in the disk drive adapter

Where My 5120 Came From

Around February 2023, about three IBM 5120’s appeared on eBay with no minimum bid. One of them stood out to me because the description stated the system wasn’t fully working. I never liked the idea of cannibalizing a vintage system (especially one in clean condition and “almost working”), but I had in mind that perhaps some parts from that 5120 might be used to complete another 5110 system that I had (specifically, the Display card). Or, vice versa: perhaps the not-quite-working 5110 could get this 5120 going again.

But I had very little space left in the house to dedicate to my own hobby, especially since I knew the 5120 is quite a large system. The 5120 is so large, it really needs two people to safely move around without some kind of portable hoist. On top of that, the set also included the original IBM printer – which is also large. Such a system was either going to have to block the front of a fireplace or consume the space on a dining table. That’s fine for a single person but intrudes on space that others within the family may want to use.

While I had a lot of technical interest in the 5120, I had hoped they’d find a more suitable home — especially since it was over 1000 miles away from me, so shipping would be both risky and expensive. But since these systems don’t show up often on public auction, I didn’t want to miss the opportunity on a reasonable price. I had decided a budget limit and stuck to it. In the end, I ended up outbid and was content that fate had been decided.

A few weeks later, one of the 5120 sellers contacted me, since the higher bidder ended up not paying. Not only that, the system was actually shipped across country, but then the bidder refused delivery and it got shipped back (unopened).

eBay intentionally makes it difficult for buyers and sellers to directly communicate with each other (by blocking e-mail addresses, web addresses, and phone numbers in there messaging system) . Somehow we got past this (after crossing paths at VCF East) and I was able to talk with the seller directly through e-mail. I learned more about the history of the system, such as this particular 5120 was used by KR Electronics (located in New Jersey, see here) for the typical roles of accounting and payroll, along with some in-house design software. It’s neat being the “2nd owner” of a system and knowing a bit about its usage history.

I tried to help the seller find a buyer — such as someone who already had a 5120 (and might be familiar with restoring it) or was already in the process of restoring one. After several months, nothing came together for this seller, and so we negotiated a deal, and I got delivery of the system in early June 2023. The documentation included with the system has been fascinating, with IBM bulletins as late as 1988.

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