Timeline for What is this chip enable circuit doing? (old TTL, with diode, with pull-up?)
Current License: CC BY-SA 4.0
10 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Apr 4 at 19:53 | comment | added | BZo | Fair enough-- thanks. I'm a bit puzzled by why the Signetics data book hides the ball a little bit on that, but that certainly makes the overall circuit make a lot more sense. Thanks for your help. | |
| Apr 4 at 19:49 | comment | added | hacktastical | found a schematic here: github.com/bbenchoff/2513CharGenAdapter/blob/main/Schematic.pdf | |
| Apr 4 at 18:59 | comment | added | BZo | Whoa whoa, where do you see that? The only data sheets I find just call it "chip enable" with no bar or asterisk indicator. If that's the case then this all makes way more sense to me. | |
| Apr 4 at 18:56 | comment | added | hacktastical | I should add that the 2513 CE input is active low. Thus, the gate input being low forces the char gen value onto the output, otherwise the light pen is allowed to control it. So it's a bit more useful to think of the function as being a negative-logic OR. | |
| Apr 4 at 18:54 | vote | accept | BZo | ||
| Apr 4 at 18:53 | comment | added | BZo | Got it. Thanks. The light pen input is actually always disconnected (they never made the light pen lol). The "A" signal comes from bit 7 of the data bus, and part of my confusion here is that "and" behavior here actually does not make any sense to me, but that's a different analysis I need to do. Thanks for the help on this one. | |
| Apr 4 at 18:43 | comment | added | hacktastical | The diode is there to allow "A" to toggle low when the gate is high. It blocks the gate high-drive. | |
| Apr 4 at 18:34 | comment | added | hacktastical | Yes, it forms an AND. What seems to be happening is when that gate is high, it's allowing the light pen to manipulate the character gen CE - perhaps to make a tracking cursor. | |
| Apr 4 at 18:10 | comment | added | BZo | Thank you. Two questions: one, confirming that I guess my original vibe was right that chip enable can only go high when both the gate and A are high? Feels like I might call this a "wire-and" instead, no? Two, why the diode? What is the current flow we are needing to prevent in which case? | |
| Apr 4 at 18:01 | history | answered | hacktastical | CC BY-SA 4.0 |