| ??? 11/27/06 14:27 Read: times |
#128543 - TTL outputs Responding to: ???'s previous message |
The original TTL 74XX chips and the subsequent 74LXX, 74SXX, 74LSXX, 74ASXX and 74ALSXX all had outputs which could sink ten times more current (down to ground) than they could source. This lead to the widespread use of pullup resistors to provide additional source current and faster low-to-high transistions. Once the pullup resistors are used, it becomes inevitable that tri-stated signals must be active low, so that there are no spurious active pulses when all drivers are disabled. |
| Topic | Author | Date |
| why do we use negative logic in processors&control | 01/01/70 00:00 | |
| Speed | 01/01/70 00:00 | |
| Some previous thoughts | 01/01/70 00:00 | |
| Wire-AND-ing is the remaining benefit | 01/01/70 00:00 | |
| Just imagine ... | 01/01/70 00:00 | |
| ah well... | 01/01/70 00:00 | |
| That would cause one to avoid their devices | 01/01/70 00:00 | |
| because pullup | 01/01/70 00:00 | |
| But why? | 01/01/70 00:00 | |
| TTL outputs | 01/01/70 00:00 | |
| Remarkable advantages when switching to GND | 01/01/70 00:00 | |
| more current | 01/01/70 00:00 | |
| Yes, but why? | 01/01/70 00:00 | |
Physics | 01/01/70 00:00 |