Shop OBEX P1 Docs P2 Docs Learn Events
resisotr value for prop demo board can I substitute value? — Parallax Forums

resisotr value for prop demo board can I substitute value?

mikedivmikediv Posts: 825
edited 2010-03-07 19:09 in Propeller 1
Hi guys quick question I am putting together a prop demo board for the VGA circuit it calls for the usual 240 ohm resistors and the 470 ohm ones as well I ran out of 240 ohms resistors but I do have plenty 270 ohm will that work ok
I figure with the 5 percent tolerance the 240 could be as high as 250+ anyway but before I solder them in I figured its worth asking
thanks

Comments

  • hover1hover1 Posts: 1,929
    edited 2010-03-05 01:35
    You could use two 470 ohm in parallel to get 235 ohm. A little closer.

    Jim
  • tonyp12tonyp12 Posts: 1,951
    edited 2010-03-05 01:46
    270 will work, the colors will be just a tiny bit dimmer.

    The color that only use the pin with 470ohm will be as normal.
  • mikedivmikediv Posts: 825
    edited 2010-03-05 01:56
    To be honest guys I am building the prop pocket term I bought a bare board and I building it up so it will just be used a serial terminal. Hover I thought about that but I dont want to Smile up the board with a ton of resistors,,
    Tony when you say a little dimmer will I notice it for terminal display?? and if I did have to do graphics will I be disappointed I didn't wait for the 240s thanks guys
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-05 11:20
    Because of "what's in the bits box", I have only ever used 470 + 270 Ohm combinations.

    It will give a few percent lower output but that will be nothing compared to the differing outputs of whatever monitor it ends up on. At least it errs on the low side, so there shouldn't be any overloading / clipping problems. This way of generating a 0.7 Volt, 75 Ohm feed is not pure, but works perfectly well. The VGA and composite outputs are just about the right standard for hobby / commercial usage.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Style and grace : Nil point
  • hairymnstrhairymnstr Posts: 107
    edited 2010-03-05 13:15
    I've built a couple of VGA driver circuits with the Prop and I've only ever used 470 and 270 and they work fine. Colours are good and crisp on my monitors too.
  • mikedivmikediv Posts: 825
    edited 2010-03-05 15:11
    Thanks guys, Toby my friend good to see you ,, can I ask what awesome project you are "hand building for the present" lol What ever happened with the Dram stuff????
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-05 17:21
    No new ironings, for a while now. The SDRAM has lapsed back to EDO thoughts but then I realized the problems that Dr_A had in getting the static+latches to work in the first place. What with work (hateful stuff ) and a constant desire to·fall asleep ( because of the hateful stuff ), I haven't done much.

    So hows the DracBlade coming along ??

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Style and grace : Nil point
  • mikedivmikediv Posts: 825
    edited 2010-03-05 20:43
    Toby Dr-A "my hero" hooked me up with partially built board it works just fine, the funny thing I just cant let go the fact that the other one still doe snot work , so I put them side by side one problem is DR-A sent me a newer version than the one I built but
    none the less I have gone pin by pin trying to find the fault my old board works just fine even lets me load Mbasic program but after almost exactly 45 seconds I get the error we have been talking about,, you have the let the board rest before it will work again
    its not thermal I have replaced every single chip including the prop, I have bypassed the power supply and just used a regulated supply to feed the board the 5 and 3 volts never fluctuate they are rock steady I just cant figure it out

    Toby if you see this what do you think about a cap being bad or something like that?
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-05 23:31
    mikediv

    45 secs would sound like a thermal thing, but the fingers and nose would soon find the effected bits. There is the possibilities that something is overflowing leading to a crash.

    I left one running for over a week, not meaning to, and it just waited at the cursor. I'll have a think, tomorrow.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Style and grace : Nil point
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2010-03-06 02:59
    I've used a high-current supply to find/sort board faults more than once.· There's a tip from the bench.

    Post Edit -- Resistors have tolerances, carbon films are usually 5%.· That's something that you have to spec and design for.· It may seem like a small detail, but that's good engineering.· The values for a 240 and 270 at 1.05 and 0.95 tolerance,·respectively, are very close.· If you want to be a stickler for accuracy, buy metal film, 1% or better.

    Post Edited (PJ Allen) : 3/6/2010 3:11:07 AM GMT
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2010-03-06 05:27
    I've used 270R for all of them - I doubt you would even notice the difference. Variations between monitors seem to be greater.

    Re mikediv and resets, I've had a couple of boards do that too. Only the older ones, and I'm still not sure if it is the propeller or the external ram. One test would be to put the pocketerm program on the board, maybe change pins if needed and see if that crashes.

    I have a sneaking suspicion there might be a timing loop that is just on the limit, and I've very much hoping that the latest board with its shorter paths and less potential crosstalk will be 100% reliable. I've had one running for 3 days now.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.smarthome.viviti.com/propeller
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-06 08:30
    The only useful thing that time and poverty shows you is that for the vast majority of projects, pasive components can be "close enough" and you will get it working. Only in a few cases must things be exact, and the due to tolerances, even 1%, a tweek will be required.

    I once had to play with an old IVC three tube camera, it had precision resistors with variables on the end. The designers calculator must have said "we need 62.87K here" so they had 60.3K fixed, custom made, resistors with a 5K variable set about halfway.

    I bet their CD collection was in alphabetical order too.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Style and grace : Nil point
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2010-03-06 08:43
    Yes, and I'll bet the labels on the tins in their cupboard all faced the front.

    Re resistors, my dad reckons back in the 1950s you could buy 2% or 5% or 10% and they were very accurate. How so? Well, they just measured them when they came out of the factory. The ones within 2% went in the 2% pile etc. The 10% ones were guaranteed to be either +10% or -10%.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.smarthome.viviti.com/propeller
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-06 17:06
    I can remember the large "stick" resistors, they were about 2" long bits of carbon rods with wires wrapped around the ends. The values were depicted by "Body", "Tip", "Spot". At 2Watts they had plenty of surface area to get rid of the heat. The old capacitors were wildly inaccurate, 50% errors on electrolytics were common ( and they were the size of a small dustbin )
    That would stop anybody building a computer in a matchbox.

    Happy days

    Back to reality ---- I still haven't formed any ideas as to why the DracBlade should start, run for a set 45 secs and then demise other than Thermals or its running out of something in a loop. You could remove bits to see if the fault is there from the bare min system of 2 latches (for 64KB). This the back of "Baby", the Prop is under the '138 and EEPROM, and the ram is under the two latches. I couldn't get the paths much shorter using DIP chips.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Style and grace : Nil point
    PICT0834.JPG
    640 x 480 - 52K
  • mikedivmikediv Posts: 825
    edited 2010-03-06 23:05
    Hmmm Dr-A I am having the problem with older board the new version you sent ma has been rock solid I have left it on for days with out a hiccup,, Toby its defiantly not thermal first thing I thought of but your right fingers test fine so does a thermal gun the hottest chip is 82 degrees don't forget I am running a separate external power supply because I thought the regs were acting up ,, DR_A do you think a slight design fault? the funny thing is I can duplicate the failure over and over it does not matter what is running Mbasic or just CPM
    Guys when I boot CPM and let it sit a the prompt is it static at that point or is the prop still running stuff???? I can look at signals with scope but when it locks up I can not find what is changing the difference between the old version board and the new while slight is enough so I Can not compare pin for pin to the working board... DR_A has been awesome and I should just give up on the old board but I am so curious as to why because everything looks so darn good I just wish I could go WOW found it its this or that but I have to admit I am stumped


    Toby I will always admire your talent that board is pretty impressive for home built DIY
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-07 00:15
    Yeah, ironing - douce point.

    Photographic skill - nil point.

    There is always the "fresh eyes" approach. You could send it to me, and whilst I promise nothing, sometimes the problem is obvious. Work permitting I'll willingly have a look at it ( the Bu****s have already got me working next weekend. I was creeping around HER to let me go to Maker Fair, in Newcastle. Hey Ho )

    I'll invoice Dr_A.

    I am up in the Midlands, only the PCB would be needed, so the postage shouldn't be too bad.

    (PM'd you)

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Style and grace : Nil point

    Post Edited (Toby Seckshund) : 3/7/2010 12:23:35 AM GMT
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2010-03-07 13:55
    It's deuce, just deuce.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-07 15:35
    Considering I spent a few months over in France, I should know how to write "12". I just thought that "nil point" was a slightly less down version of "Whats the Use ????"

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Style and grace : Nil point
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2010-03-07 17:46
    "12", in French, is douze.

    If we're talking tennis, then http://www.merriam-webster.com/dictionary/deuce
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-07 19:09
    More ignorance demonstrated, Hey-Ho. I'll plead that it has been three and a half decades since I had to write it.

    Its got me wondering, are there many French Prop users ?????

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Style and grace : Nil point
Sign In or Register to comment.