Roger, That is where the situation gets 'sticky'. It all depends on what the circuits at the end away from the K3 have implemented. You have two pullup resistors on each line and 2 separate voltage sources.
1) If the voltage to the pullup resistors in the internal device do not match the voltage to the pullup resistors in the K3, there will be a continuous flow of current through both pullups on any one line when the drivers are at their open, non-conducting state). If the voltages are close to the same value, this may not cause a problem, but if they are quite different (say one is 12 volts and the other is 5 volts), that could create a big problem. 2) The drivers in the K3 (when at the low level) must carry the current provided by both pullup resistors. Whether that is a problem or not depends on the value of the pullup resistors and the current capability of the drivers. In my view of a good design, open collector (or open drain) drivers are the only thing to use. The terminating device can determine what voltage is needed for reliable switching at its input and that device controls the high level by providing its own pullup resistors. Secondly, the pullup values can be sized according to the high level input current required by that device and can control its immunity to noise by controlling its input conditions. All is under the control of the external device designer. Unfortunately, much of the ham gear available uses bare, open inputs and depends on the driving device to provide the voltage and levels it needs at its inputs. 73, Don W3FPR 73, Don W3FPR Roger wrote: > Mike Fatchett, > OK, but what happens when you use them with devices > that already work without them,e.g. Array Solutions 6 Pack and early band > decoders? > Roger K8RS > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
