RCA COSMAC 1802
RCA COSMAC 1802
The very first computer I ever had was a "homebrew" machine based on the RCA 1802 microprocessor. I didn't build it, but rather was given it on loan so I could study and practice machine-language programming in 1977. It was very much like many other 1802-based kits and homemade computers of the late 1970s, having a couple of hexadecimal displays and a bank of toggle switches to manually enter data into memory.
The 1802 computers of the 1970s were based on a design that was published in the August 1976 issue of Popular Electronics magazine. That was followed by a series of articles in 1976 and 1977 that described updates and improvements in the basic design. The COSMAC ELF, as it was called, was operated without any operating system or built-in monitor ROM for a user-interface. Programs were entered directly into memory with toggle switches.
 Popular Electronics August 1976 Issue |
 COSMAC ELF, built from Popular Electronics Articles |
One thing that struck me about the ELF was its total - almost pure - simplicity. It is as "close to the machine" as one could get. No peripherals, no keyboard, not even a rudimentary "monitor" ROM with basic utility functions. Just a processor and RAM.
Lots of other manufacturers came to market with various small computers in 1976 and 1977, including the KIM-1 and Sym-1, the Apple II, the Commodore PET and the Radio Shack TRS-80. And of course, Altair and IMSAI computers were available with disk and other peripherals, add-on memory and CP/M operating system support. But these systems were all much more expensive than the ELF, which could be built for under $100.00.
On the other hand, even the least expensive bare-bones offerings from other microcomputer manufacturers had useful programming already installed in the form of a monitor ROM, which had lots of useful higher-level functions built-in. Most of them also had a form of BASIC on ROM, which made programming much easier. But the 1802 systems of the day had none of this kind of support, so I think this probably hurt their popularity. Most hobbyists that owned them really didn't do much with them.
Everyone I knew that had one would turn it on, enter a dozen or two bytes into memory - forming a simple machine language program - and then flip the switch to "run." An LED would blink or the displays would count, and that was it. Seemed like that's all anyone ever did with them. But there was a lot more a person could do with the ELF, and a lot to like about the 1802.
The main thing I liked about the 1802 was that it was made in CMOS, and that allowed it to work with very low power. That made it good for embedded controllers, especially battery-powered devices or those used in environments that might have intermittent power and might need to run on backup batteries for while. It has four digital input lines and one digital output - directly connected to pins on the 1802 chip - which make simple sense and control possible. And it even has a built-in DMA controller, which allows peripherals to access memory without processor intervention.
As an aside, the COSMAC acronym came from RCA's description of the 1802 integrated circuit CMOS technology as "complementary silicon/metal-oxide semiconductor." They also referred to the processor as "complementary-symmetry monolithic-array computer," or COSMAC.
Here's an odd duck - an IBM COSMAC 1802 - maybe IBM's first microcomputer. Probably not, but it was made before the IBM PC because this one is from 1980:
 IBM COSMAC ELF keypad and display |
 IBM COSMAC ELF, powered on |
 IBM printed circuit boards of the 1980s had unique traces |
 Internal wiring of the IBM ELF |
Quite a rare ELF, that one. More like an ELF-II, having a keypad and extra 7-segment displays. I built it in 1980.
I kept it "bone stock" and never upgraded it with any add-on options. I say that, but I did add one small upgrade for a while. I cut the trace to the memory chips, and installed a rechargeable battery with steering diodes. It made the memory non-volatile, and the NiCad would keep the memory alive for months without a charge. But after many years, the battery failed and rather than replace it, I removed the steering diodes and ran a wire from the power supply to the memory, like it was designed. That's the large red wire in the photo above.
I said earlier that 1802 systems of the late 1970s lacked upgrade options, such as peripherals, an operating system, utilities or user-interface programs. But that's not entirely true. Aftermarket manufacturers were perhaps late to support the 1802, but that may have been because the market was just moving so rapidly.
For example, in 1974, you really couldn't buy anything for any microcomputer. They didn't exist, at least, not in any form that the average person could actually purchase. But by 1976, you could buy one of a half-dozen systems and by 1978, many microcomputer systems had disk drives and operating systems. They could do almost anything a minicomputer could do, and some had popular language interpreters and/or compilers and even applications software.
That's about the time when small companies like Netronics and Quest Electronics began to offer 1802-based systems and peripherals that worked with them. Netronics, for example, made the ELF II, which was a lot like the IBM COSMAC, shown above. It had 256 bytes of RAM, a hexadecimal keypad, two-digit hex display and a single "Q" LED. It also had five expansion slots.
Extending the buss with expansion slots made the ELF II a very capable computer. The owner could start with a modest investment, buying essentially a bare-bones ELF without any hardware or software for under $100.00. Then add upgrades as time and funds permitted.
ELF II Upgrade Hardware:
ELF II Software:
As I said above, the 1802 seemed to me to be an ideal processor for embedded controllers. So I shouldn't have been surprised to learn that it was used as the flight computer on space missions. The 1802 was available in a Silicon on Sapphire version that was radiation hardened. That plus the fact that it used very little power made it attractive as a microcontroller used in space. Six 1802s were used as the brains of the Galileo probe, which launched in 1989 and orbited Jupiter between 1995 and 2003.
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