TTL computing

From: Peter C. Wallace <>
Date: Wed Apr 10 10:00:29 2002

On Wed, 10 Apr 2002, Tony Duell wrote:

> > The main reason I have never played with them is I could never find a
> > Fuse Prom Burner schematic that looked reasonable. I still would like to
> That's probably because fusible PROMs are notoriously difficult to 'blow'
> correctly. They're also getting hard to find now, and as they're strictly
> one-time-programmble, I really wouldn't want to use them in a new design.
> Some of the modern electricallly eraseable technologies are getting
> pretty fast -- certainly fast enough for experimental processors. And
> they're a lot easier to program. OK, the devices are massive by
> comparison with the old bipolar PROMs. but they're also not expensive (at
> least not compared to 2 or 3 of the old bipolar fusible-link PROMs, and
> you need to reckon on buying at least 2 for every one you will use as you
> will make programming mistakes), so it's worth 'wasting' most of the
> capacity of the chip.
> > do a TTL computer with fused based proms ( or EEPROM's as modern
> > substitute )for control logic. I am just finishing up a nice FPGA design
> > but thinking this is going to be a pain to get a serial prom and have it
> > burned too. 50,000 sure ! quantity 1, HA-HA you must be kidding.
> > (EPC1441LC20 altera 440,800 bits -- any place in Canada). While TTL is
> > low density you don't need to pay $$$ for a license for modern
> > programiable logic, have the software needed 5+ years down the road! (
> > That is assuming TTL is still around 10 years from now )
> For experimental and educational projects I much prefer TTL (including,
> of course the CMOS versions of the TTL chips, like 74HCxxx parts). It's
> easier to prototype with, easier to test (you can clip the 'scope or
> logic analyser wherever you like), and easier to see what's really going
> on. It's quicker to make small changes to the circuit as well (on an FPGA
> design I did about 5 years ago, a full compile of the main chip took
> overnigh (OK, PCs have got faster since then, but FPGAs have also got
> larger!)). That meant every small change took a day to test. A soldering
> iron and/or wire-wrap tool is a lot faster for changing a few connections
> :-) You also aren't tied to a proprietry program running on some
> computer/OS that I don't have...

        Actually It's not that bad, recompiling my Sweet16 16 bit CPU takes
only about 5 minutes using Xilinx tools (on a fast machine though) The tools
are available free (webpack). The tools do require Windows, but some people
have reported success using them under WINE on Linux.

> Please don't attempt to convince me that FPGAs make more sense for
> production. I don't need convincing of that...
> >
> >
> > > > The PDP11/05 was 2 full hex-height boards just for the CPU, so around 200
> > > > chips. It used mostly TTL, but also some PROMs containing the microcode.
> > > > And the TTL included chips rather more complicated than just gates --
> > > > things like 16*4 RAMs, 4 bit latches, multiplexers, etc.
> >
> > This was ball park figures. A 4 bit shift register ,4 bit up counter
> > ,dual 4-1 muliplexer would be the typical ALU parts. I was looking at
> I wouldn't call those SSI parts.
> > 74HCXX chips as 74LSXX is harder to find and lots more power. How ever a
> > 74LS382 style alu would be used rather t
han a 74LS181 if LS parts were
> Why? The '181 has many more operations, some of them useful....
> > used. A 8 bit x 1 register file is a 8 bit addressable latch and a 8/1
> > multiplexer. The alu design is for a undefined 24 bit processor with a
> Sure... I guess 74x170s are really hard to find now :-(.. All these
> wonderful chips I grew up using are discontinued :-(

Nah, there 100's of them in a $19.00 FPGA...

> -tony

Peter Wallace
Received on Wed Apr 10 2002 - 10:00:29 BST

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