Hallo Ethan, > But how are the specs? It's an easy-to-get chip, but is it _really_ > a replacement, or is it just close? The MC3446 is nothing more then a Open-Collector buffer going out and an inputbuffer: |\ | \ B 0-------- -----+ | / | |/ | +---------0 X /| | / | | A 0--------- -----+ \ | \| The main difference of these buffers and normal TTL-buffers is their behavior when powered off. With TTL-buffers you run the risk that they tend to pull the lines low when powered off, the IEEE-buffers won't. Not that much but it is measurable. Connect several 1541s to a C64 but leave one unpowered. You most probably won't notice it. Power off more and there will be a point that the chain won't work anymore. For techies amongst us: the pull-up resistors have their share in this behavior as well. And talking about pull-up resistors: the 3446 also contains such a resistor PLUS a diode. This diode makes sure the resistor start pulling the line (L) when no power is applied. Take http://www.funet.fi/pub/cbm/schematics/computers/pet/8032/8032029-02.gif as example. UB16, a 6520, takes care of the databus of the IEEE-bus: Port A reads the data, port B outputs it. I cannot do without the OC-buffer. That's why I need the 7407. This is a very common TTL-IC and will cost about 50 dollarcents ??? At least we're not talking about dollars. As Port A already serves as an input, we don't need one. If you want a kind of protection for your 6520, you could place a 74LS541, an 8-bit buffer, between the bus and the 6520. FYI: I connected a bi-directional PC LPT-port directly to a 8032 and 4040 and used the PC as a second drive without any problem. > If so, then you are probably limited to one or two peripherals. What makes you think so? The IEEE-bus has much more then a normal fan-out (= output capacity). I don't know the fan-in (= input load) of the 6522 right know but it is peanuts for the bus. ___ / __|__ / / |_/ Groetjes, Ruud \ \__|_\ \___| http://Ruud.C64.org Message was sent through the cbm-hackers mailing list
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