Basic Homebuilt Discrete DTL gate circuits (Diode Transistor Logic) (C) 1997 Copyright by Richard Steven Walz for Non-Commercial Use Commercial Rights Reserved by Author rstevew@armory.com Chris Nohr wrote: >theory etc. However, when I started into electronics, all I could find >in any books about actual physical logic gates were vague references to >Microprocessor IC's etc. So my question is, can you actually get >physical AND NAND OR NOR X-OR etc. gates, or are they only available in >batches as IC's? -------------------------------------------------------- We can surely take care of that! The only really serviceable transistor gates that are stand-alone and hobbyist-ready are the inverter-based gates, tis true, but we need to add a few things before we have something like a gate that you can count on and not have to fiddle around with it and the problem of successive array levels from logical input (left) to output (right) on a page or standard diagram. The crucial thing for the most feasible homebuilt gates is that they have guard diodes on all inputs so that one input doesn't feed backwards into another input and the output it came from. The next is seeing the efficacy of using open collector logic to simplify things, and these were some of the early six inverter chips used for both 5 volt and 12 volt logic in industry and computing. Here is a sample: o +5 VDC ONE TRANSISTOR INVERTER | Or TWO-INPUT NAND GATE o +5 VDC / 2-3 1N914/1N4148 DIODES | \ R2=~1K ohms 1 10K OHM RESISTOR \ / ^ 1 1K OHM RESISTOR (opt.) / R1=~10K ohms \ | 1 2N2222/2N3904 \ Collector = <--(optional) TRANSISTOR / /-----------+-----> OUTPUT | Base |/ w/ R2: active gate output INPUT A >-------|<|-----+-+--|>|--| w/o R2: open collector output D1=1N914/1N4148 | D3 |\ INVERTER Option ---->= \v Q1 NPN XSTR: 2N2222 or 2N3904 | Emitter | INPUT B >-------|<|-----+ _|_ D2=1N914/1N4148 - GROUND Note that an INVERTER is simply this schematic without INPUT B and D2. Now this works fine as an open collector NAND gate, which allows the construction of all OTHER gates from it! Likewise an NOR gate does, but it is not as simple to build as a NAND gate. All other gates can be made with as few as four or five of these gates. An invertor is merely the gate using only one input, or tying the two inputs together. An input can be considered HI if not pulled LO, but it's state is really that of the off or tri-state, seen later with true TTL gates of 4 or more transistors. It pays to optimize the resistors used for this design to your transistors, and then use the same in all such gates as proof of the principle, as good gates should be interchangeable and not require adjustments of this circuit if it is to constitute a true hobbyists logic family. This is a variation on what is called DTL or Diode-Transistor-Logic that preceded TTL. The optional pull-up resistor to 5 Volts or Vcc is what would give this gate the full measureable HI output capability, rather than merely regarding the open condition as HI by default. As you can see, the LO is all that can really be asserted to the next input of another gate, as it pulls the next transistor's Base bias LO shutting off the transistor and allowing the collector to be open and non-grounding, so it could be pulled HI if need be. This remains true today in modern TTL gates and much of all logic. Seldom is a true HI used in bipolar logic, but it is in CMOS and other MOS. The real reason for a pull-up is for a definiteness of logic level, rather than leaving a bad connection "float" which is hard to troubleshoot! A Second ASCII-matic of the Inverter/NAND Gate in DTL Discrete Components Predecessor of TTL O +5V Diode-Transistor Logic = DTL | +------+----+ | | \ \ <------- Remove for Open- NAND / 10k Ohm / 1k Ohm -Collector Output Gate \ \ 1N914/ Option | DTL | (in)<----|<|------//-----+ INVERTER +-------->out | or NAND | 1N914/1N4148s | |/ in<----|<|-------------+---|>|---| NPN 2N3904/2N2222(A) 1N914 |\v _|_ - Of course this can also be implemented by great or tiny relays of any kind: An ASCII-matic of the Inverter/NAND Gate in Diode-Relay Discrete Components Relay Boolean Gate Logic O +V | +------+---------+ | | \ Coil resistor \ <----- Remove for Open-Collector NAND / 470 Ohm (?) / 1k Ohm Output Gate \ \ And most relay gates ARE O.C.! 1N914/ Option | Relay | (in)<----|<|------//-----+ INVERTER +-------->out | or NAND | 1N914/1N4148s | | in<----|<|-------------+---|>|---+ | 1N914 | | \ _|_ Relay Coil /---> ___ N.O. (Normally Open) \ | | | _|_ _|_ - - -- -Steve -- -Steve Walz rstevew@armory.com ftp://ftp.armory.com/pub/user/rstevew Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!! http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public