| ??? 08/02/05 14:41 Read: times |
#98687 - Comments... Responding to: ???'s previous message |
Hallo Charles,
some comments to your circuit, I recall here: 
- Signal "SELIN" also needs a pull-up. - D1 is needed to prevent, that supply voltage of printer port electronics drives current into the target's supply. For the same reason 74VHC244 must be used, not 74HC244! 74VHC244 does not contain this obligatory protection diode from each input to Vcc, as 74HC244 does. - Unfortunately, if inputs of printer port electronics have pull-ups connected to their supply voltage, then even via outputs of 74VHC244 a current can flow into Vcc of target, because at outputs again protection diodes to Vcc are present! A simple way to make Vcc of target to go down to nearly zero volts, is to connect a low ohmic resistance from Vcc of target to ground. I successfully used 150 Ohm. - I used an additional 4k7 pull-up at RST-input of micro and connected !EA/VP-pin directly to Vcc (with my AT89S52). Kai |
| Topic | Author | Date |
| ATMELISP programmer and software project | 01/01/70 00:00 | |
| Similar project | 01/01/70 00:00 | |
| proposals for hardware modification | 01/01/70 00:00 | |
| 11/13 changes you suggested done. | 01/01/70 00:00 | |
| Simple things | 01/01/70 00:00 | |
| Hmm... | 01/01/70 00:00 | |
| And where was it? | 01/01/70 00:00 | |
| 8052.com | 01/01/70 00:00 | |
| very nice! | 01/01/70 00:00 | |
| Thanks | 01/01/70 00:00 | |
| Ah I see | 01/01/70 00:00 | |
| No hmm | 01/01/70 00:00 | |
| Not you, the author | 01/01/70 00:00 | |
| Comments... | 01/01/70 00:00 | |
| All your suggestions implemented. Tks. | 01/01/70 00:00 | |
| aren't you gouys doing this the wrong wa | 01/01/70 00:00 | |
Is provided | 01/01/70 00:00 |