You don't ned no pullup for +5. All open TTL inputs are reading High w/o any pullup.

Pete Turnbull wrote: Yes Pete, not all People over here are totally braindead. I've told him that he can leave out the +5V Connection for testing purposes, for nothing other.

That has nothing todo with your Commodore problem. Regards, Holm

On 2015-Feb-07, at 11:20 AM, Holm Tiffe wrote: No you didn't. While the context of the discussion is testing and that may have been your intention, your comment specified no such qualification, and as such at best left it ambiguous/unclear. Pete's comment was valid clarification and additional information (although I could have minor quibbles with some of the technical phrasing).

Anyways (for Noel), everyone's right: - An open TTL input will generally function or act as a logic high. It was not unknown for designs to leave inputs open for static-state high, but it is considered poor practice and can result in problems in some circumstances.

On 08/02/2015 11:20, Holm Tiffe wrote: Ooh, somebody's still grumpy today. Not me though, I'm writing this with a smile on my face :-) Yet it will sit in the archive for years and since the message itself says nothing about that, it could easily be taken out of context by someone who knows less than you, I, or Brent. Moreover, it doesn't always work as a reliable test, simply because there /are/ circumstances where letting an input float will sufficiently change the operating characteristics. No, you're wrong. Try looking in the Texas Instruments TTL Data Book; in the Fifth European Edition, it's clearly described on pages 5-4 to 5-5. It's also mentioned in chapter 3 of my 1971 edition of the Texas Instruments book "Designing with TTL Integrated Circuits", published by McGraw-Hill, which also has a lot to say about switching speeds, DC and AC noise margins, and noise rejection. You might also care to read Chapter 9 especially section 9.06 "Some comments about logic inputs" in "Horowitz & Hill "The Art of Electronics" (one of the standard textbooks) culminating in their analysis of unused TTL inputs left open, and hence having /zero/ noise margin. -- Pete Pete Turnbull

Pete clarified your potentially misleading statement. He wasn't "pissing on your feet". Sometimes Holm, attempts at redemption just result in digging yourself in deeper.

On 2015-Feb-10, at 1:21 PM, Holm Tiffe wrote: It's not likely that you would have encountered that scenario and it's not likely you will. That doesn't mean anything. While there are some faults that may be considered typical or common, many are simply unique. They are unlikely up to the instant they occur and then they continue to be unlikely (to occur again). Over the decades (as I recall, I first played with TTL in 1972) I have observed that when semiconductors fail - especially when of their own volition and not due to external trauma - they can fail in very strange and bizarre ways. I once encountered some failed ICs that would work after heating them up with a soldering iron and then revert to failure again - not after a few minutes of cooling off - but days or weeks later. I once encountered a common JFET that one day decided it would become a sensor for changes in the surrounding electrostatic field. The equipment would intermittently hang up until power-cycled. It was eventually correlated to the activation of a neon indicator, triggering a nearby JFET to latch up in conduction. Opening the equipment moved the bulb an inch or two away from the JFET, and now it was fine. That was hell to figure out and realise what was going on. No it wasn't noise on supply lines, it was mere physical proximity. Many would have just shotgun-replaced components until the thing worked and put it in the category of "bad transistor" without having any idea of the unique nature of the fault. Was that fault scenario likely? Will I, you, or anyone encounter that again? Not long ago I encountered a similar situation as the OP: traced a fault to a two-point circuit (a PROM IC output driving a data input of a 4-bit TTL register, well, 3-point including a pull-up resistor). After eliminating the R as a problem, it was difficult to discern whether the output or the input was the fault source and I don't like hacking up PCBs or making scattershot, unnecessary parts replacements to find a fault. I assessed as I could and eventually decided it was the PROM, probably swayed that way as it would seem the more likely part to fail. So I went to a lot of trouble to build programmer hardware, write control software, source the ROM contents, burn and install the new PROM. The PROM output wasn't the problem. The input was the faulty point. I wish I had made more measurements. The OP's situation was such: unexplained semiconductor failure. While it was more probable it was something in the oscillator I wasn't about to make presumptions. When advising on repairs from a distance I try to cover the range of possibilities to pursue a course that will make accurate eliminations, and not go down a path that may falsely eliminate from consideration even unlikely failures (which testing the input only open-circuit could have done). Like my PROM, the OP's xtal osc is not a readily sourced part and checking the input might have been fortuitous.

I could write a reply noting your error(s), or suggesting you've not paid any attention, or considering list etiquette and trimming posts but as I expect everyone else is bored by now I'm tempted to summarise with a single word (my last). Plonk.

You don't ned no pullup for +5. All open TTL inputs are reading High w/o any pullup.