grant@hp-pcd.UUCP (grant) (03/17/85)
As to the likelihood of hurting the 7406, forget it. The thing is open-collector; there is only an NPN to ground, so shorting its output to ground is NO problem. (to +5 would be a different story.) As to the current, a pull-up resistor on the line is all that supplies it. I'm not looking at the schematic, but no more than 1-2 mA could flow. Frank's reset is fine. The addition of one-shots would only be for debouncing the switch (or making sure that if it's reset it is REALLY reset). Grant K. Garner hp-pcd!grant
@RUTGERS.ARPA:prindle@nadc (03/22/85)
From: prindle@NADC I realize that the proposed modification to a C64 to add a reset switch represents the optimal electrical approach. However, the output of U8 is the RESET line proper and appears on the expansion connector, the user port, and the serial bus connector. By simply grounding this line externally (with a switch connected to one of those reset lines coming out and ground), you get a reset effect. Sure you're shorting the output of U8, a hefty 7406, and sure, it draws a bit of current, but this is no problem as long as you don't sit on the switch for more than a fraction of a second (wired or's have been shorting out 7400 gates for ages). My 64 is 2 years old (a bit of a record for 64s) and going strong and I use such a reset button daily. And it doesn't even void your warranty, so if it does break your 64, just turn it in for a new one. Most commercially available reset buttons just hook to one of the external connectors also, although the more sophistocated ones probably drive the reset line low with a one-shot so it can't be held low indefinitely. Frank Prindle Prindle@NADC.arpa