On 11/25/09, Alexandre Souza - Listas <pu1bzz.listas at gmail.com> wrote: No microcontroller required - it's already just a handful of TTL chips. -ethan

On 11/25/09, Alexandre Souza - Listas <pu1bzz.listas at gmail.com> wrote: Because I can wire up a handful of chips in the time it takes to design and build a microcontroller then write and debug the program for it. If I have to design a complete functional replacement, that's one level of effort. If I can clone an existing design, that's a different level of effort. In the case of a 4X MR78, I can easily redesign the ROM field and slap on two upper address switches and not worry about the timing between an IM6100 and an independent microcontroller. This device needs to sit on a wad of ROM and listen to the VT78 ask for more bytes and feed them down the pipe. It would need to have enough I/O lines to address the ROMs in the box and to listen to the VT78 - I haven't analyzed the design, but for 16K of ROM, you need either a lot of free I/O pins to strobe the address pins of the ROM or you need to throw in a shift register or something similar to externally address the ROM, adding to the complexity of the software design. Either way, it's an entirely new peripheral at that point, with lots of debugging ahead. Building some with 8-10 TTL parts is a construction project much more than a design project. Someone who designs hardware all day is free to go build an AVR-based MR78 replacement, but I don't know that I would ever find the time to spend to take that route - I've worked with design engineers on testing and refining commercial products - it's easier than it used to be, but it still takes a lot of time and effort to ensure that what is happening is what you want to happen and no more, no less. It's not that I never work with microcontrollers. I have built several AVR-based devices and am building an 8051-family device. Those items require flexibility far beyond clocking a bunch of ROM reads. They have to interpret command streams from larger processors and have complex behavior (well over "here's your next byte"). They need a microcontroller. I'm not sold on the design-time/construction tradeoff for a peripheral for a VT78 - the market for that is small enough I could probably hold all the devices in one hand - not worth putting large amounts of effort into designing something completely new. Pick your battles and your toys. Sometimes, a 555 LED flasher is easier and cheaper than a microcontroller even though you can do it either way. -ethan

On 11/25/09, Dave McGuire <mcguire at neurotica.com> wrote: A dozen or two? Probably. This is a much smaller scale than that. How about 4-8? -ethan

Whatever, I do like to design with microcontrollers :) Call me lazy! :oD

Tony Duell wrote: ... and one time two tubes did all your work @ 45 volts. :)

Firstly I object to this idea that every digital project has to include a microcontrolelr (in fact that's why I stopped reading Circuit Cellar magazine!'. Yes, microcontrollers are very usedul, and I use them myself when I consider them to be appropraite. But not for everything Now lets look at this device. From what I understand it has to step through the contents of an external EPROM under the control of some signals from the VT78. The outputs of the EPROM are fed back to the VT78 (possily after going through a buffer). So the microcontroller would replace the address counter and handshake logic. You need a microcontroller with a fair number of I/O pins for that, proaly a 8751 or something. If you don't have a programmer for that, you need to hanf an EPROM off it containing the firmware (which then means an address latch) But even if you do, it's a 40 pin chip to wire up. Five TTL chips are peoably only twice as many pins to wire, if that.. Then there's the thign that I can wire pins (and design TTL circuits) a lot faster than I can write microcontroller firmware. Period. Of course once I've written the firmware, I can program aas many microocontrollers as a I want, but this is hardly going to e a large-volume project. I think I could wire up the necessary TTL chips in less time that it would take me to type-in read-written source code, assembly it and program it into a microcontroller. -tony

Very much so. For one-off projects the overall size of the PCB or stripoard is not that important. Well, I must admit that 'my favourite language is solder' :-) The problem comes when you want to run external (to the microcontroller) stuff at those speeds. You then _have_ to design the whole thing as a high-speed digital circuit, and an awful lot of programmers don't know how to do that. I've seen the resultant mess all too often... PErhaps it's becuase I think at a very low level al lthe time, but I tend to cosider the total number of transistors (or gates) as a measure of complexity/elegance, not the number of individual packages I have to solder to the PCB. This does not mean I don't use microcontrollers. Of course I do. In front of me there's a thing I built that takes in 8 bit parallel data and outputs a string of pulses (it actually simulates the output of a barcode reader 'wand' [1]). To do that in TTL would be a lot of chips. But the comples stuff fits in an 18 pin PIC, there's a little glue logic to ensure the Cnetronics interface timing is correct under all conditions. The right tool for the job. Stiting on top of it is an interface box to link 2 different parallel ports together. It countains 4 or 5 TTL chips. To do it in a microcotnroller would IMHO be the wrong thing to do. It would be slower (although that might not matter) and you'd still keep a couple of the TTL chips which are just uffers from one port to the other. The result would be aout the same component count, but more complex, harder to debug, and piossibly less reliale (my experience is that while a single complex chip is possibly more reliable than the many chips it could replace, a complex chip is less relaile than a simple chip. AndI would think a PIC would be less reliable than the '74 and '00 that it would replace on this board). [1] Actually to download software from a PC into an HP41. I have a modified HP82153 wand with an optoisolator wired to the wand output signal inside the HP41 module. This box connects to the LED side of the optoisolator, the PC sends data to it, and the HP41 thinks somoody has run the wand along a line of barcode. And before somebody tells me to get an HPIL module, I have one, OK... -tony

Well, I am not going to bet on it, because I have much the same superstition on gambling as John Scarne, namely that I won't gamble in any week that contains a friday :-). But my experience is thatr the larger (as in transistor count), the chip, the more likely it is to fail. And FWIW I've got plenty of (plain) TTL chips that are gettign on for 40 years old and still working fine. I suspect there'd be a very raal chance of a flash memory microocntroller developing bit-rot before then. You may be right, I don't know, But I _would_ bet that in 40 years time it'll be a lot easier to find data on TTL chips than tracking down the person who wrote the firmware for the microcontroller and convinicing him to give you a copy so you can replace the a dead chip. -tony