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The VT100 terminal already had an 8080 processor.

So why did it have to be connected to a host computer? Why not use its processor to perform computations?

Is it because the VT100 processor's purpose was only to transmit input from the keyboard to the host computer, receive & display the corresponding output to the screen and to render the command line interface on the screen?

Was it also because the host computer was much more powerful? If yes, how powerful was it compared to an 8080?

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  • Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Retrocomputing Meta, or in Retrocomputing Chat. Comments continuing discussion may be removed.
    – Chenmunka
    Jun 17, 2023 at 8:28
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    There are some dozens of processor inside the device you are currently using, anything that can be programmed can be called processor and many are more powerful than that old computer, some HDD literally have 3 different ARM processors. But still why do you have the CPU? Jun 17, 2023 at 10:24
  • Because you wanted to work on the host computer and the vt100 was a very good way to do that. Compare to e.g. a adm3a which wasn’t as powerful. Jun 17, 2023 at 23:00

9 Answers 9

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"Having a processor" is not sufficient for something to be a general purpose computing device. The VT100 was designed for a specific purpose, and adding more features like storage and I/O would have made it more expensive for a purpose that most customers would not want or need.

At the same time (late 1970s), other people were indeed building general purpose CP/M machines with 8080 processors (eg. IMSAI 8080 was an early one, there are many others).

So, the answer to "Why didn't DEC do this?" is essentially "Because DEC's customers did not want that."

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    To summarize: the purpose of the Video Terminal 100 was to be a video terminal, not to be a stand-alone computer.
    – RonJohn
    Jun 14, 2023 at 6:33
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    Could perhaps also add to the en "nor need that". They already had mini- and mainframe-computers (like PDP-11) to do the actual computing - what they needed was an alternative to the teletypewriters when it came to communication with this computer. And when the terminal additionally had the ability for some simple graphics (block graphics) and could make things like actual menus, that was all they really needed. Jun 14, 2023 at 21:13
16

The 2MHz Intel 8080 ran at 290 thousand IPS (instructions per second), whereas the VAX 11/780 that a VT100 plugged into ran at 1,000 thousand IPS (aka 1 MIPS). Thus, 3+x faster.

Just as importantly, the VAX had much higher IO throughput than anything designed around the 8080.

And, of course, the VAX was 32-bit, while the 8080 was 8-bit. Even slow PDP-11 systems were faster, because of the better IO capacity.

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    Actually, I'd say that the IO was more important than the raw power. After all, once you had four people using it, each person had potentially fewer MIPS than with a dedicated 8080. Of course, a VAX-11 instruction was a full 32 bit operation and so was more powerful than an 8080 instruction.
    – JeremyP
    Jun 14, 2023 at 8:37
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    @JeremyP Yes, unless its about very narrow cases, I/O thurput is what defines a computer. That's why /370 are still a thing, they are designed around that principle all the way from instruction set to OS. Also, I can imagine way more VT100 being attatched to low end PDP-11 than upper end VAX :))
    – Raffzahn
    Jun 14, 2023 at 8:43
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    The VAX was also a multi-user system where the business records you were trying to access were stored.
    – David42
    Jun 14, 2023 at 18:40
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    The power of IO to make a machine feel fast is amazing. I've had great fun using a server as a desktop. Lots of folks focus on faster CPUs, but faster CPUs are useless without faster and faster IO.
    – chicks
    Jun 14, 2023 at 19:22
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    Yes. Try running a database transaction ON an 8080 down RS232 (in those days an Ethernet was a thick yellow cable with vampire transcievers and a NIC -- Card not Chip -- cost as much (or more?) than a VT100.
    – nigel222
    Jun 15, 2023 at 11:02
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So why did it have to be connected to a host computer? Why not use its processor to perform computations?

Because a VT100, or terminals in general, are dedicated devices for a specific purpose. Having a processor doesn't mean it's maent to be a computer.

The very same thing why one doesn't play DOOM (*1) on a washing machine, a fridge or your cars engine. All of them got at least one processor (*2) but are not computers.

The same way the purpose the VT100 Terminal was build for is to be a Terminal, not a computer.

In fact, I'd say that today, with most applications build around the web, most PCs are used almost exclusive as terminals - after all, a web browser is nothing else than a somewhat bloated ... err ... advanced terminal application.

When writing this question, you didn't use your PC (*3) as a computer, but as a terminal to connect to the massive computer network we run RC.SE on (*4).


Heck, that question has even potential for recursion:

  • Why does the PC need an 8088 when there is already an 8048 Keyboard controller?

  • Why does it need that 8048, when there is already an 8048 within the keyboard?_

  • Even more, there's another one in the mouse!

Bottom line: Having a processor is in next to all cases just a way to replace hardware (circuitry) by software, enabled due the universal nature of processors, but the purpose is usually not creating an universal device.


*1 - Yes, I know, it's a fun to make such devices play DOOM anyway :))

*2 - If not a whole lot more CPUs

*3 - Or whatever device you wrote this question with.

*4 - Well, kind of :))

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    "In fact, I'd say that today, with most applications build around the web, most PCs are used almost exclusive as terminals - after all, a web browser is nothing else than a somewhat bloated ... err ... advanced terminal application." - ironic, that: full circle, except with more software involved therefore more opportunity for bugs to ruin your day ....
    – davidbak
    Jun 14, 2023 at 14:14
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    @davidbak Jau. Back in the late 70s when first 'intelligent' terminals had luxurious functions to manipulate the character buffer like inserting lines, clearing only regions or changing attributes, I remember several programmers telling me that noone would ever need such complex functions.They would only result in more bugs.
    – Raffzahn
    Jun 14, 2023 at 14:24
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    They weren't wrong.
    – davidbak
    Jun 14, 2023 at 14:30
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    @Therac - yes, and? a) how does that change the premise that "most PCs are used almost exclusively as terminals"? b) in your experience with any browser with "a DOM manager, javascript engine, WebGL, audio and video codecs, persistence layer, context search" have you experienced more or fewer bugs than if you didn't have that software? Because ... I know my answer to that!
    – davidbak
    Jun 14, 2023 at 15:56
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    "The very same thing why one doesn't play DOOM on a washing machine, a fridge or your cars engine." Or... I'm remembering "Dark Star" from the early 1970s, and a philosophical discussion with a bomb that's far too smart for anybody's good. I wonder if John Carpenter realized that scene was prophetic at the time he wrote it?
    – nigel222
    Jun 15, 2023 at 11:07
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Digital did make a standalone VT100, but it didn't use the built-in 8080. It used a 12-bit Intersil 6100 CPU chip and could run PDP-8 programs such as WPS-8. It was called the Digital DECmate VT278.

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    They also made the VT180 – terminals-wiki.org/wiki/index.php/DEC_VT180 – which ran CP/M. A VT100 could be upgraded to a VT180. Although it sounds like it added a second 8080 to run CP/M instead of using the VT100's one??? Jun 15, 2023 at 21:13
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    @SimonKissane most likely because of too little ram left over in the main cpu so easier to add another one instead of introducing memory banking Plus performance is guaranteed. Jun 17, 2023 at 23:09
  • Slightly later, DEC released the VK100 “GIGI” (1982), a home-computer(ish) form factor terminal that was also an 8085 computer that could run BASIC.
    – scruss
    Jun 19, 2023 at 1:14
  • @SimonKissane - the VT180 was a VT100 plus an add-in Z80 card; the terminal processor remained a terminal processor. I'd guess that logically it operated like a VT100 connected to a separate Z80 microcomputer.
    – dave
    Jun 19, 2023 at 9:01
  • The DEC Rainbow seems to also have been both a terminal and a computer. Jul 9, 2023 at 15:32
9

At the beginning of the '70s it was more the limitations of memory than the processing power that was what was a terminal or what was a computer. You can see it when you study what early glass teletypes are made of. Even the Apple I used shift registers instead of RAM for the glass teletype part of the circuit. The datapoint 2200, from which the 8008 was derived, was relatively compact even with its TTL CPU but its 2K of memory was of the serial shift register. The computer revolution of the 70s/80s has more to thank to the progress in memory technology than to CPU's imho.

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    The VT100 was released in 1978; hardly the "early 70s".
    – RonJohn
    Jun 14, 2023 at 15:42
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    I remember a terminal that had core memory. It was spooky - you could turn it off and turn it back on the next day, and your output would still be onscreen unchanged. Jun 15, 2023 at 3:47
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    @MarkRansom core memory was awesome, for that very reason. My grandfather's company had an NCR single-user accounting computer with core memory. If the power went out, you could restart most programs by looking at the output (it was a teletype-style computer), going to the assembler listing in one of a giant set of books to find the hex address and then punch the buttons to start running from that address.
    – RonJohn
    Jun 15, 2023 at 8:14
  • @RonJohn: What protection did systems have to ensure that a read cycle would not be started unless the system would have enough stored energy to complete it?
    – supercat
    Jun 16, 2023 at 17:59
  • @supercat good question. Capacitors?
    – RonJohn
    Jun 16, 2023 at 19:38
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What would you do with it? Its only connection was an RS-232 serial port, which connected it to a mainframe or minicomputer. It didn't have a disk for program storage, or even a cassette interface like a TRS-80, a popular personal computer of the time.

It was about the same size of a TRS-80, so they could potentially have turned it into a personal computer, but then it would be much more expensive. DEC made most of their money from sales of computers, which needed terminals. They'd still need terminals for those customers, and turning them into full-fledged microcomputers would not make them significantly more useful in that role.

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Sharing

This is hinted at in other answers, but it really is the key. There were two ways that a video terminal (or a Teletype, for that matter) could be used:

  • Single User

A terminal could be a computer. The Datapoint 2200 is one of the earliest examples. At its heart it was a terminal. But plenty of things could be added, including a hard disk drive, floppy disk drive, network card and additional RAM, so that it could indeed be used as a standalone computer. The 2200 used one processor, which was the basic design for the Intel 8008, to run everything.

A terminal could also include a computer in the same box. An example of that is the Heath H-89/Zenith Z-89 which used a Z-80-based video terminal as the housing for a full CP/M (and other OS) computer. For a non-technical user, it was one magic box. For a technical user it was a computer and a terminal combined together.

Many other computers, commonly running CP/M but not exclusively, had one box (or sometimes more than one) with a computer and a terminal, such as a VT-100, connected via a serial port.

In all of the above cases, this was one user = one computer.

  • Multi-User

This was the primary use case for VT-100 terminals and, I would argue (but don't have the stats to prove it) for most serial terminals. A single computer, from PDP to VAX to various microcomputers (8080 or Z-80 with MP/M, 8086 and above with MP/M-86, Concurrent DOS, PC/MOS and other operating systems, plus numerous other systems from Data General, Alpha Micro and others) could connect to anywhere from 2 to dozens of serial terminals. While a multi-user system could be used as a bunch of nominally separate user spaces, sharing the cost of CPU/hard disk/etc., the big advantage of a multi-user system was (and still is today, though more commonly in the form of a networked system) sharing files (instead of having to physically transmit or copy on a floppy disk, etc.) and sharing applications (e.g., accounting, inventory tracking, etc. all using the same data).

If you are DEC then you want to (a) sell more PDP and VAX systems with lots of terminals along with the systems and (b) sell lots of terminals to competing multi-user system vendors by building a high-quality, standard setting terminal such as the VT-100.

Many large multi-user system vendors sold either their own terminal or a rebadged version of someone else's terminal. WYSE eventually made some of their own computers, but their main product for many years was terminals that could be connected to other vendor's computers and emulate the native terminals, including DEC terminals.

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  • 1
    The Datapoint was predated by the Cogar 4 by about half a year. And there were others, mostly forgotten by now.
    – Raffzahn
    Jun 14, 2023 at 14:30
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    It was as well conceived as a terminal with local editing capabilities. Like being able to enter data records, store them on tape and send them on request to a mainframe. A replacement to classic remote entry stations where users had to first punch cards and stack them in a reader for transmission - results turned back as cards to be punched or printed locally again. Like the Datapoint it was soon discovered that local processing could be done as well. Unlike Datapoint this was supported by the company right from the start. Like keeping the return record for offline on screen handling.
    – Raffzahn
    Jun 14, 2023 at 19:11
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    A key observation is that most computers are used for information processing tasks far more than for actual computing tasks. An order processing system where every agent has a VT100 that's plugged into a VAX 11/780 will be much more useful than one where every agent has an I7-based computer with four gigs of RAM and a terabyte of hard drive space, but none of them are connected to each other.
    – supercat
    Jun 14, 2023 at 20:24
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    An early company I worked at flipped that script - they used an all-in-one CP/M machine as a terminal to a PDP-11. I forget why they preferred that to a dedicated terminal. Jun 15, 2023 at 3:55
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    @MarkRansom a guess could be downloading files Jun 17, 2023 at 23:11
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The vt100 VT 100 specs had a nice display, a nice keyboard, a RS232 interface, a slow processor, 8 KB of program in ROM and 3KB of RAM. You could probably use it as a calculator without RAM and a storage unit.

Always remember that technical stuff is always a compromize between conflicting goals. In this case, price of additional resources and the concept that a terminal should be, exactly, a terminal connected to something bigger.

At the time, 1978-ish, a Megabyte of Ram costed about 10.000 USD Ram prices. A small harddisk probably about 500 dollars storage prices

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    My gut feeling is that a harddisk in 1978 was more like 5000 USD. The linked list seems to be removable media and there an RK-05 cartridge is listed at 6120 USD for a whopping 2.4 MB. You needed the drive for it also, that price is not listed.
    – UncleBod
    Jun 16, 2023 at 7:14
  • @UncleBod: I agree. You could probably get a floppy drive for 500 dollar in 1978 but not a harddrive (I bought my first computer in 1979 but could only afford a casette tape drive).
    – ghellquist
    Jun 16, 2023 at 12:36
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So why did it have to be connected to a host computer? Why not use its processor to perform computations?

What wasn't mentioned here before was the other direction:

Many computers were JUST computers, that came with storage devices (sometimes not even that), but without any devices for human interaction. So the host computer needed a terminal to be connected, and often several terminals where connected to the same host.

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  • Disk drives and printers are most certainly IO. Thus, “with storage, but without IO devices” is nonsensical.
    – RonJohn
    Jun 17, 2023 at 17:27
  • Substitute "human interface devices" or "interactive peripherals".... Also, printers and disk drives are peripherals, just as graphics cards and keyboard controllers are. Jun 17, 2023 at 22:40

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