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RE: [N8VEM-S100:2419] Re: 6502 CPU Board V1 components question



From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of Vince Mulhollon
Sent: Monday, February 17, 2014 12:51 PM
To: n8vem...@googlegroups.com
Subject: [N8VEM-S100:2419] Re: 6502 CPU Board V1 components question


On Monday, February 17, 2014 11:38:46 AM UTC-6, Fabio Battaglia wrote:

I don't have 1K/1.2K SIP-10 resistors, but I do have enough 1.5K, do you think those would be OK too?

I obtained a copy of the Wilcox book to go with my 68000 board build and this kind of thing is discussed at length (pages and pages of equations) early in the book, from memory less than 500 ohms could hit the overcurrent limit for his design and more than 8500 ohms would undercurrent as a pullup depending how thirsty the inputs were for current.  Lower switches faster but wastes power, higher uses less power but switches slower.

I ran the numbers myself using a real live Motorola 74LS682 datasheet and you've got two criteria:

1) Too low of a resistance to Vcc and a TTL L output will overcurrent and catch fire.
So figure the data sheet Vol = 0.5V and Vcc at 5 V, thats 4.5 volts across the resistor.  And the 74LS682 (or most any LS) will catch fire around 24 mA so 4.5V/0.024A = 187 so less than 190 ohms to Vcc could burn out a LS output.  Thats assuming there's no load on the TTL output other than the resistor, which would be pretty pointless.  So I'd keep it WELL above 200 ohms.

2) Too high a resistance to Vcc and the input current of 0.4 mA will pull a false low.  So figure 5 volts Vcc, half a volt for ground bounce and half a volt for noise, that leaves 4 volts, then Vih for 74LS is 2.0 volts, so the resistor must drop less than 4-2 = 2 volts.  At the previously defined 0.4 mA.  So 2/.0004 = 5000 ohms.  Thats assuming it only drives one input.  So I'd keep it well under 5000 ohms.

So, well over 190 ohms and well under 5000 ohms.  How bout a geometric mean?  (5000*190)**0.5 = 975 ohms so ideally it would be around 1K which is probably where the designers are getting 1K from.

How bad is 1.5 K?  Well, lets get a percentage of the range, from at the low end where it catches fire and the high end where it eternally outputs a false Low level. and call that full range 100%.  That 1500 ohm pack is (1500-975)/(5000-190) = about 11 percent high of center of the safe range.  Now you were probably building stuff with 10% tolerance resistors so the range of plus or minus 5 percent higher or lower means the possible ranges just barely don't overlap by about a percent, so there should be a measurable effect.

But will the measurable effect matter?  Well, worst case is your 10% resistor could be 5% high (or 5% low, which doesn't matter) and you've decided to use something that centers 11% high, for a theoretical peak of 16% high of perfectly centered on a 100% scale.  16 outta 100 is a long, long way to go until it fails in eternal false L level.  So you're all good, by far.

What if you got the idea of throwing two 1500 resistor packs in parallel?  Well that would be 750 ohms.  (750-975)/(5000-190) is 5 percent low, so its technically better, however, look at the failure mode... Too low means it burns out permanently, but too high doesn't hurt anything.  So you're closer to parallel two, but in a dangerous direction, and further away but in a safer direction to just use one.  Combined with two of them costing twice as much, I'm thinking, don't parallel them.  Also the component you never install can never fail or have a soldering issue or add to capacitive load, so its twice as reliable, sorta, not to install parallel resistors.

TLDR don't worry about 1500 ohms, on the other hand, I wouldn't go trying 4700 ohm packs..

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