Datapoint 2200
The Datapoint 2200 was a very early personal computer, first prototyped in April 1970, pre-dating the appearance of the microcomputer (and the microprocessors which powered them). It was an major product from Datapoint, just after their first product, the Datapoint 3300 (which was a video terminal).
The base unit included a Central Processing Unit, a small amount of main memory, two digital cassette magnetic tape drives for for mass storage, a keyboard, and a video display, using a CRT, capable of displaying 12 lines by 80 columns. Eventually a wide variety of peripherals were available for it, including an industry-standard magnetic tape drive, disks, modems and a printer.
There are two different stories about the motivation for the 2200 (possibly both true). One is that the founders of Datapoint had always been interested in producing a personal computer, but started with the video terminal as it was an easier first step. The other is that the 2200 was intended to be a 'programmable video terminal' (so that a single hardware design, with appropriate firmware, could emulate many different kinds of terminals); however, customers quickly learned that they could program it, and it tapped into a hitherto unseen market.
Microprocessor plans
The original concept was for its CPU to be a custom chip - what eventually came to be the microprocessor. None existed yet, so CTC contracted with Intel to produce one; that chip that eventually resulted was the Intel 8008 (just about the first microprocessor ever). Due to a funding crunch at Datapoint (and possibly also a personnel crunch at Intel), the chip was delayed, and so the 2200's CPU was actually constructed out of discrete SSI chips. The intent was to eventually replace the original CPU with the single chip version (probably to reduce the unit cost), but the microprocessor was slower than the existing CPU, so the plan to use a microprocessor never happened.
Implementation
The 2200 is a 8-bit machine; the internal registers are mostly 8 bits wide, as was the ALU. Instructions all have an 8-bit operation code, and an optional single byte of immediate data, or two bytes of address.
It had the capability for interfacing with an extensive array of external peripherals, for communications and other functions, such as mass storage.
Version I
The 2200 was initially produced and sold with what was called the 'Version I' machine. When planning for the 2200 started, semiconductor RAM did not exist yet. Datapoint was using large shift registers in the 3300, and it was decided to use them for main memory in what was later named the Version I machine, just as the earliest computers had used delay lines. Fairly naturally, the design which emerged to work with that was a serial computer; an advantage to that was that it would use fewer components.
It had 28 instructions, 7 registers, and a 7-deep (possibly 15-deep; sources differ) stack, for subroutine calls. It could handle up to 8K bytes of main memory (the Program Counter was 13 bits wide); configurations were available with 2K, 4K, 6K, or 8K bytes.
Version II
When semiconductor RAM became available, the CPU was re-designed to be a parallel unit, producing the so-called 'Version II' machine, released in early 1972. An interrupt system was made available for handling peripherals.
It had 31 instructions, 14 registers (2 sets of 7, for 'Alpha mode' and 'Beta mode'; two instructions could switch back and forth), and a 16-deep stack (the stack can hold the H,L memory address register values as well as subroutine return points). It could handle up to 16K bytes of main memory (with a 14 bit wide Program Counter); configurations were available with 4K, 8K, 12K, or 16K bytes.
External links
- 2200 - documentation at Bitsavers
- Oral History of Victor (Vic) Poor - contains background to the creation of the 2200
- The Texas Instruments TMX 1795: the (almost) first, forgotten microprocessor - also covers the 2200 and the intention to use the 8008
- Oral History Panel on the Development and Promotion of the Intel 8008 Microprocessor - contains information about the attempt to build the Intel 8008 for the CPU
- Vintage Computers - large collection of images
