In this blog post, I restore a Teleram 4000, a portable vintage computer from 1983 that ran CP/M and used Magnetic Bubble Memory.
Motivation
Anyone who has read this blog a few times knows my fascination with computers that used magnetic bubble memory. The technology was only in use for a few years, and the computers that used it were fairly rare. The Grid Compass is a prime example, and another computer that I have restored. I previously restored a Teleram Portabubble, which is a much larger computer with a CRT screen. The Teleram 4000 is more of a notebook-sized portable, similar to a TRS-80 Model 100 in size.
Teleram created two computers in the series, the T-3000 and the T-4000. As far as I can tell, the primary difference is that the 4000 offered an 8-line display whereas the 3000 only offered a 4-line display. Both computers featured 64KB of RAM, a Z80L CPU, and either 128 KB or 256 KB of bubble memory. The computer has a serial port, and typically would come with a built-in terminal program.
Teardown
The batteries were undoubtedly shot and the charging voltage is unknown and undocumented, so I decided to tear it down before powering it up. First, I removed the case top:
We can’t see the bubble memory or the CPU because they’re under the keyboard, but we can see several lead acid batteries. The cells are 2V / 2.5 AH. There are a total of 6 cells configured as two packs. Here are a couple more pictures:
After removing the batteries, I removed the keyboard, and we can get a look at the bubble board:
I removed the bubble memory board and took a closer look:
The bubble memory is an intel 7110-4 along with the 7220 chipset, a very common bubble memory subsystem for the day. Interestingly, there is a second socket, along with drivers for a second bubble memory. It would be relatively easy to add a second memory device and extend the capacity to 256 KB. The only wrinkle would be the need to format the new bubble memory and write an operating system image to it. The bubble memories do not work as independent devices; I think their data in this configuration would be interleaved.
Under the bubble memory board, we can finally see the main board:
There are two ICs that are obscured by double-face tape. They are a 27C32 EPROM that holds the bootstrap and a Z80L microprocessor. There’s also 64KB of DRAM, a keyboard controller, and a bunch of traditional logic. All together, it’s relatively simple.
Bypassing the batteries
The battery configuration is interesting. There are two independent banks that are a nominal 6V each. These banks are connected in parallel most of the time. However, when the bubble memory is accessed, they are switched in series to provide the 12V supply to the bubble memory. After a short time delay, they switch back to parallel. This creates a bit of complication. We can’t simply power the computer with a +12V supply, because the 6V supply is actually used to power the main logic. My best solution was to power the computer by wiring up two independent power supplies to the battery connectors.
Each supply I powered with +6.2V with a 1.4 A current limit. Make sure to use independent supplies with no common ground, otherwise you’d be up for a rude surprise when they were put into series. My tests showed the computer consumed up to about 1.4A on the 6 volt supply (across both packs, when in parallel). When it switched into series mode, it would typically pull 100ma from the second pack.
It runs CP/M
The computer runs CP/M 2.2
and there are a handful of programs present
Most of the stuff here is standard CP/M stuff such as STAT, PIP, DDT, ED, etc. We have the assembler, ASM, and the loader, LOAD, as well. There are a few Teleram-specific programs such as TTALK and ASSIGN. TTALK is a terminal program that can be used to connect to external devices over the serial port. I used it in my demo video to interact with a Votrax Personal Speech System (PSS).
PIP may be easily used to transfer ASCII text by sending it to LST: or LPT:, or reading it from RDR: or PTR:. RDR: is by default assigned to the keyboard/LCD, so you’ll need to “STAT RDR:=PTR:” first to make RDR: use the serial port.
There’s also a CONFIG program that I think will allow you to configure some of the settings, including possibly the assignments for the devices like RDR:. The ASSIGN command also allows you to remap devices, in a more granular way than STAT.
My github repo contains disassemblies and reverse-engineered manpages for TTALK, ASSIGN, and a few of the other programs.
Dumping the Bubble and the Boot ROM
I was worried about the possibility of damaging the bubble by accident. With no external floppy drive and no Teleram corporation to send it in for repair, that would render the computer unusable. As such, one of my first goals was to dump the contents of the bubble memory.
I used Claude Code to write a Z80 assembly program to read all the sectors on the A: drive and print them in Intel Hex format out the serial port. I then captured them and converted them back to a 128KB binary image. This should be sufficient to restore a bubble memory if the need ever arises. The disk image is in my github repo.
I also made a copy of the 4 KB Boot ROM and I had Claude Code disassemble and comment the source code. The Boot ROM includes a full monitor that is triggered by NMI or by an unreadable bubble memory on boot. How you trigger the NMI yourself I don’t know — it might be as simple as a special key combination.
Adding BASIC
The disk image did not come with Microsoft Basic (MBASIC). In fact, there wasn’t even enough free space to hold it. I had to delete DUMP.ASM and the various text files (AFILE.TXT, FILEA.TXT, FILEB.TXT, etc) to make enough room. After that I reverse engineered the TTALK protocol, and I used TTALK to send the binary MBASIC.COM over to the computer. This allowed me to write basic programs and demo them in the video.
The reverse-engineered protocol and a python-based send/receive tool is included in my github repo.
Teleram vs the TRS-80 Model 100
| Feature | Tandy / Radio ShackTRS-80 Model 1001983 | TeleramT-30001983 | TeleramT-40001983 |
|---|---|---|---|
| Released | April 26, 1983 | February 1983 | November 1983 |
| CPU | Intel 80C85 (CMOS)2.4 MHz | CMOS Z80 (Z80L) | CMOS Z80 (Z80L) |
| RAM | 8K – 32K | 64K | 64K |
| ROM | 32KOS + applications | 4K | 4K |
| Display | 8 × 40, text240×64 pixel graphics | 4 × 80, text only | 8 × 80, text only |
| Operating system | Proprietary ROM menu OS | CP/M | CP/M |
| Mass storage | Battery-backed CMOS RAMno internal disk | 128K bubble memoryexpandable to 256K · CP/M drive A | 128K bubble memoryexpandable to 256K · CP/M drive A |
| Bundled software | BASIC, TEXT, TELCOM, ADDRSS, SCHEDLin ROM | MBASIC, assembler, teleTalkno word processor | not documented |
| Modem | Built-in, 300 bauddirect-connect | Novation Cat (acoustic)external, included | not documented |
| Other I/O | RS-232, parallel, cassette, barcode, system bus | RS-232C, teleConnect bus | RS-232C, teleConnect bus |
| Battery | 4 × AA, ~20 hr+ internal NiCd memory backup | Sealed lead-acid pack | Sealed lead-acid pack |
| Price (1983) | ~$1,099 – $1,399 | $2,995 | not documented |
Display is the only confirmed difference between the T-3000 and T-4000 — the T-4000 doubles the line count to 8 while keeping the full 80-column width. CPU, memory, bubble storage, I/O, and battery are identical.
T-3000 date: introduced February 1983 (in development since May 1982); one contemporary review placed its market debut in mid-1982.
Model 100 price: ~$1,099 (8K) to $1,399 (24K) per Wikipedia; an entry price near $799 is also widely cited for launch.
T-4000: CPU, RAM, ROM, bubble storage, I/O, and battery confirmed identical to the T-3000 from hardware inspection. Bundled software, modem, weight, and price remain undocumented.
In short, the Teleram 3000 / 4000 are much more capable computers than the Model 100 series. However, the capability comes at a cost in both money and weight, and this led to dominance of the Model 100.
Conclusion
In conclusion, this is a really fun little CP/M computer if you can find one. The batteries are still available, albeit expensive, so it could probably be restored to true portable operation if you wanted to. It should be possible to expand the bubble memory, though it would take some effort to write a new OS image. Expanding the bubble memory would make it a lot more usable, probably provide sufficient room to load a few games (Zork!).
There is an external drive that I’d love to have, if anyone has one available. I would also love a copy of the technical manual. There’s an external bus connector that is not documented.
