Homebrew computers

I understand some people here are part of a homebrew computer club. I came upon this website from a link one of their members had on their site. I wanted to learn more about building a computer, so I decided to come here. I can already tell this is an excellent website, and I imagine I am going to learn a lot here.

I have a background history with digital ICs, (TTL) and an understanding of what computers do and how they work. I have been programming since I was 6 or 7, so I'm no stranger to the technology field. However I have never built a computer. As my name implies, I admire Steve Wozniak for his ability to think, then design a computer.

I have a few questions targeted toward those of you who have been a part of building a computer from the component up.

What do you have to have in order to build a computer? By that, I mean how do you know which chips, ICs to use, etc? Are there core (really important parts) of the digital logic ICs, for example, are there certain ways to align up chips so that they act as a processor? What do you have to do in order to build a cpu? From looking at various websites in the webring, some people have used nothing but TTLs in their computers. Others have built CPUs for their computers. Until I stumbled upon the webring, I could never really find anything other than "how to build our own computer" websites that showed you how to select an existing PC motherboard, and put in various ATI and NVIDIA video cards, to build a "customized" Windows PC.

Any and all help will be extremely appreciated.
Wozinator

The ALU slice, SN74181 is the place to start. This chip implements 16 different logic functions of two variables. You need two of them to make an 8 bit wide Arithmetic Logic Unit. You need some meory to hold the instruction words and a state machine to fetch the instruction words, decode them, and execute them, then do it all over again.

I admire your intestinal fortitude for wanting to tackle such a daunting task in this era of very large scale integration.

BTW, Woz started at a somewhat higher level. He had the 6502 Microprocessor to start with. He and his team exercised considerable creativity in developing the hardware and software interfaces that surrounded the 6502.

Hats off to you , most people yank out hair out just attempting to build a digital alarm clock from discrete gates

Many years ago I had the "8088 cookbook" which showed exactly what you are after, right from programming with diode matrix, up to adding serial ports etc. I would never have built the computer in the book, there were to many good microcontrollers around that worked better but it was good reading.

I would think about building a computer with an existing microprocessor, in fact I've looked into it, but there are so many pins on them that I wouldn't even know where to begin to decipher how to work with or build for said architectures. I have a Cyrix floating around in my basement in a box, and a couple Pentiums. If you would suggest I work or build for a microprocessor, which one? And would the faster/newer the processor make it more difficult to work with? If so, might I just go with what you said, the 6502? Jameco doesn't sell those on their website, I checked. But they do sell 6800, and 68000 processors.

https://jameco.com/webapp/wcs/stores/servlet/CategoryDisplay?storeId=10001&catalogId=10001&langId=-1&categoryId=1065

Thanks,
Wozinator

And btw, Paul O'Brien, I checked eBay for the 8088 cookbook, but it wasn't on there, so I bought 2 other books on them. Thanks for the tip.

Cheers
Wozinator

Cheeez... If you think a microprocessor has a lot of pins (40) then a discrete TTL design will absolutely knock your socks off. A working discrete design should be in the neighborhood of 100 16 pin Packages. That's 1600 pins for you home gamers. The last one of these I worked on in about 1972 had a schematic that covered six hand drawn D-size sheets. They were held up on an artists easel so we could flip the sheets over to get to the one we wanted. I'm not sure I want to go back to those good old bad old days.

LOL

Papabravo said:

I'm not sure I want to go back to those good old bad old days.
LOL

Click to expand...

Yes lets have the good old days! Enough of these namby pamby WIMP interfaces, we want real computers!
Ones with front panels with lamps and switches so you can "toggle in" a simple program and watch it single step using the lights.
And how about using an old ASR33 teletype for your HMI, you could even store your program on punched paper tape!

Or on second thoughts....????

JimB

Wozinator said:

I would think about building a computer with an existing microprocessor, in fact I've looked into it, but there are so many pins on them that I wouldn't even know where to begin to decipher how to work with or build for said architectures. I have a Cyrix floating around in my basement in a box, and a couple Pentiums. If you would suggest I work or build for a microprocessor, which one? And would the faster/newer the processor make it more difficult to work with? If so, might I just go with what you said, the 6502? Jameco doesn't sell those on their website, I checked. But they do sell 6800, and 68000 processors.

https://jameco.com/webapp/wcs/stores/servlet/CategoryDisplay?storeId=10001&catalogId=10001&langId=-1&categoryId=1065

Thanks,
Wozinator

Click to expand...

Intel 8088 is a good start, or Motorola 6809 or 68HC11. Any more power and
you have lots of pins and speed to deal with. The Radio Shack TRS80 was an 8080, the RS Color computer was a 6809, then IBM did the 8080. All good picks (no pun).

I think I still have the 8080 cookbook and bug book in a box in the garage.
They are good reading.

mramos1 said:

The Radio Shack TRS80 was an 8080, the RS Color computer was a 6809, then IBM did the 8080.

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I thought the TRS80 was a Z80 CPU?.

If we're talking home computers, how about the 6502?, probably the most popular home computer CPU of them all?.

Opps. You are right.. Z80.. Not 8080..

With Woz in his nickname, the 6502 sounds like the chip.

mramos1 said:

Opps. You are right.. Z80.. Not 8080..

With Woz in his nickname, the 6502 sounds like the chip.

Click to expand...

Not to mention:

Aim65
KIM1
PET
C64
C16
Plus4
Tangerine Microtan 65
Apple (of course)

I've still got my original Microtan 65, and loads of 6502 books.

Nigel Goodwin said:

Not to mention:

Aim65
KIM1
PET
C64
C16
Plus4
Tangerine Microtan 65
Apple (of course)

I've still got my original Microtan 65, and loads of 6502 books.

Click to expand...

I have my Moto 6800 thru 68030 book, 8080, z80, 8080 on up books still. Just thru out an old mac plus computer. It killed me. But wife wanted it out of the garage.

damn, this takes me back. in grad school, I designed a one-hot sequencer that was driven off of an eprom. 32 instructions controlling a 16x16 bit reg file array, three busses and a bit slice ALU. did it myself while all the others worked in teams of 3. It ran at the blistering speed of 1 mip. got an A and about 5 internship offers out of it. best fun I ever had - with electronics, anyway. and I learned a huge amount about computers doing it.

oh, yeah, it fit on 2 large protoboards and was wire wrapped.

mramos1 said:

But wife wanted it out of the garage.

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I thought divorce was easy in the states?.

Wozinator said:

I would think about building a computer with an existing microprocessor, in fact I've looked into it, but there are so many pins on them that I wouldn't even know where to begin to decipher how to work with or build for said architectures. I have a Cyrix floating around in my basement in a box, and a couple Pentiums. If you would suggest I work or build for a microprocessor, which one? And would the faster/newer the processor make it more difficult to work with? If so, might I just go with what you said, the 6502? Jameco doesn't sell those on their website, I checked. But they do sell 6800, and 68000 processors.

https://jameco.com/webapp/wcs/stores/servlet/CategoryDisplay?storeId=10001&catalogId=10001&langId=-1&categoryId=1065

Thanks,
Wozinator

Click to expand...

Found the 6502 microprocessor on Mouser.
https://www.mouser.com/search/ProductDetail.aspx?R=NTE6502virtualkey52600000

Sometimes the newer faster processors are easier to deal with as with the older micros you will need an EPROM progammer, and to make life easier an EPROM emulator. You may be able to get away with EEPROM but that still leaves you with the problem of getting the first bootloader program in.

Wozinator said:

What do you have to have in order to build a computer? By that, I mean how do you know which chips, ICs to use, etc? Are there core (really important parts) of the digital logic ICs, for example, are there certain ways to align up chips so that they act as a processor? What do you have to do in order to build a cpu? From looking at various websites in the webring, some people have used nothing but TTLs in their computers. Others have built CPUs for their computers. Until I stumbled upon the webring, I could never really find anything other than "how to build our own computer" websites that showed you how to select an existing PC motherboard, and put in various ATI and NVIDIA video cards, to build a "customized" Windows PC.

Click to expand...

If you want design a computer system at lower than board level, you have at least three ways to go.

1) Use a microprocessor. This way, you don't need to design a CPU. And you can use existing software to program it. At this level, you are building a system, similar to assembling PCs by choosing boards. One difference is that you are choosing chips instead of boards. Another difference is that you will need to design the interfaces (interconnections), both internal (between chips) and external (between chips and peripherals, such as keyboards and video displays).

2) Use discrete logic. This can either be gates built entirely with diodes and transistors (for the masochist only), or the use of standard IC chips.

3) Use programmable logic. A lot of university courses on computer design now use these (CPLDs and FPGAs) for hands-on design work. You'll still need external circuitry for interfacing with other things, such as lights, odd voltages (RS-232), or analogue devices (VGA, codecs, DACs, ADCs, etc.) You can design either with schematics or with a Hardware Design Language (HDL). The leading HDLs (Verilog and VHDL) look like software programming languages. So there is one major caveat - don't let the similarity mislead you into writing software style code !

Resources for CPU design with FPGAs:

http://www.opencores.org/
http://groups.yahoo.com/group/fpga-cpu/
http://fpga4fun.com/forum/
**broken link removed**
http://edaboard.com/

Who can forget the venerable PDP-8. This little gem of a CPU had 8 instructions with 4K of core memory. It was programmed from a front panel and could talk to an ASR-33 on a 20mA current loop at 110 baud. It was introducded at the amazing price of just under....$10,000.00 The PAL assembler was a hoot, and entering the code for a PAL cross assembler on an SDS-940 time sharing system and then debugging it and making it work was one of my first jobs while I was still an undergraduate. I learned a ton doing it, at the truly magnificent salary of $2.25/hr.

One additional note. The code was given to me in longhand on a thick stack of 8.5 x 11 notebook paper, written with a felt tip pen by Ed Yourden, a famous consultant and computer systems guru, in the assembly language of the SDS-940. There were only a handful of crossouts. I've never seen that feat duplicated.

Nigel Goodwin said:

I thought divorce was easy in the states?.

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Good one. No Florida is a 50/50 no fault state, each gets 1/2 and child support.. Other States are cool though.. Maybe I will talk to her about moving .

I have most of the books in boxes at my office I pay 3 times to space for.. If Wozinlator picks one, maybe I can send some off to a good home. hehe.