Timeline for Why are two LEDs on? is 7489 broken?
Current License: CC BY-SA 3.0
15 events
| when toggle format | what | by | license | comment | |
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| May 28, 2016 at 17:28 | history | edited | user105652 | CC BY-SA 3.0 |
Fixed typo
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| May 28, 2016 at 0:55 | comment | added | user105652 | The Truth Table is correct, but it is defining both rational and irrational states. In the rational world, WE is only pulsed low briefly after ME is low and D1 - D3 are stable. ME is the same as /CS, or chip select. If it is the only IC to drive on a data buss, this pin can stay low. | |
| May 28, 2016 at 0:38 | comment | added | user105652 | @PeterBennett. This is a memory IC. Normally WE is pulsed low with ME already low. WE may be low for a uS or less in real-world conditons. Holding both Me and WE low is an irrational state, with the results you indicated. WE must go high to latch in the data. | |
| May 28, 2016 at 0:31 | comment | added | Peter Bennett | @Sparky256: regardless of the data - see the Truth Table in the National (or hopefully any other) datasheet. With both /WE and /ME low, the outputs will be Logical 1 (High). | |
| May 28, 2016 at 0:19 | history | edited | user105652 | CC BY-SA 3.0 |
Edited last paragraph
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| May 28, 2016 at 0:16 | comment | added | user105652 | @PeterBennett. Under what data conditions? The outputs are an inverted version of the data inputs and are the OC type. | |
| May 28, 2016 at 0:11 | comment | added | Peter Bennett | @Sparky256: the truth table in the National datasheet shows that the outputs will be High when in Write mode (/WE Low). | |
| May 27, 2016 at 23:55 | vote | accept | Thomas | ||
| May 27, 2016 at 23:40 | history | edited | user105652 | CC BY-SA 3.0 |
Added 4th paragraph about pdf specs.
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| May 27, 2016 at 23:31 | comment | added | Thomas | Maybe I should add that if, say, D2 is brought high with the other inputs low, then all the LEDs are off. | |
| May 27, 2016 at 23:29 | comment | added | user105652 | @PeterBennett. Not with D1 connected to V+. The outputs are inverted OC type. At present only D2 should be on. I need to check the sink current for this IC. It may not be able to drive LED's directly. | |
| May 27, 2016 at 23:27 | comment | added | Thomas | @PeterBennett: That is what I was thinking. So maybe (assuming that I am doing everything right) the conclusion/answer is that the IC is broken? | |
| May 27, 2016 at 23:25 | comment | added | Peter Bennett | Also, as I read the datasheet, all LEDs should be off when /WE is low (all outputs should be high). | |
| May 27, 2016 at 23:25 | comment | added | Thomas | Thanks for the answer. If I connect all the inputs to V+ then I get all the same pattern. | |
| May 27, 2016 at 23:19 | history | answered | user105652 | CC BY-SA 3.0 |