> the 7441/74141 having some path that would try to > draw excessive current from the inputs when Vcc is > disconnected. > > Is this a good idea to try, or is it a recipe for > trashing some HC160 counters?
Its possible. The only way to really find out, is to run an experiment. Get one of your 74141s, and tie the gnd, and leave Vcc open. Connect a 470 ohm resistors to each input. Connect the other side of those resistors to its own DIP switch. A 4 switch DIP switch is needed. Tie the other side of the DIP switches to +5V. Gnd the negative side of your 5V source. Try all 16 input combinations, and see if there's a voltage drop across any of those resistors. The inputs of old TTL was usually the emitter of an NPN transistor, so it would be reverse biased if you tried your scheme. Ideally, it shouldn't draw any current. But it may, if (1) 5V exceeds its reverse breakdown rating; doubtful, or (2) there is some protection circuit, or phantom substrate structure. In old TTL, there usually wasn't any protection circuits, but a phantom diode is possible. Only way to find out, is by experiment. If the input impedance is still high with no power, then your good to go. However, if excess current is drawn. Excessive, being more than the HC160s are rated for, or higher than you want to supply, whichever is less, then you got work to do. A possible solution (if needed) is to insert a schottky diode in series in each input path. Anodes toward the counter, cathodes towards the 74141s. put 10K pull-up resistors on each 74141 input. Tie the power side of those pull-ups to the VCC pin of its nearest 74141. This is a pseudo-open-collector arrangement. Real TTL tended to have a soft pull-up. The 10K pull-ups will tend to bring up into a safe Logic-1, when the circuit is powered. The diodes will only allow the counters to sink the inputs to 0. When the counters output a 1, the diode is reversed, and off. The pull-up will bring it to a logic 1. I suggest small signal schottky diodes, because they have smaller forward voltage drops than your jelly-bean 1N914. A logic 0 must be 0.8V or less. This may be iffy if a common silicon diode is used. A BAT46 will have a much lower forward drop. Probably ~0.3V in this use. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
