CGS XOR/XNOR logic: Difference between revisions

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Line 1: <!-- missing file [[File:cgs_photo_cgs54_xor.jpg\|thumb\|center\|px\|]] --> '''CGS54''' the '''CGS XOR/XNOR logic''' board is an add-on for the [[CGS pulse divider and Boolean logic]] module, adding XOR and XNOR functionality. Unlike a true digital gate, the module is not sensitive to specific levels, but rather to the difference between them. If the two inputs are within ~~approximately~~ approximately 1.2 volts of each other, they are regarded as being the same logical value. If they are more than approximately 1.2 volts apart, they are regarded as being different. This means that while this module will work quite satisfactorily with any logic levels, it can also be used to compare two analog signals, either positive or negative in value. If only one input is connected, the output will be the same as if two identical inputs were present. == A little on how it works == [[File:cgs_schem_cgs54v11_xor.gif\|thumb\|center\|465px\|The schematic of the XOR/XNOR Logic VER1.1]] Normally the two inputs of each op-amp are held (approximately) 1.2 volts apart via the pull-up and pull-down resistors, and the four diodes in the input circuit. Connecting two similar input voltages to the inputs will not affect this ~~replationship~~relationship, other than to move the ~~absolue~~absolute voltages up or down. The outputs of the op-amps, which are wired as comparators, will reflect this. On the XOR, the inverting input will be higher than the non-inverting input, resulting in the op-amp output being negative. The opposite will occur with the XNOR comparator. As long as the difference between the two inputs is less than approximately 1.2 volts (two diode voltage drops), the relationship between the comparator inputs will remain, and the outputs will not change. Once the two input voltages begin to differ, the diode connected between the pull-up resistor and the lower of the two input voltages will pull the connected comparator inputs down, while the diode connected to the pull-down resistor and the higher of the two input voltages will pull the connected comparator inputs up. The other diodes will cease to conduct. Once the two input are taken more than 1.2 volts apart, the voltage at the diode- pull-up resistor junction will be lower than that at the diode- pull-down resistor, and the relationship between the comparator inputs will be reversed, i.e. on the XOR, the inverting input will be lower than the non-inverting input, resulting in the op-amp output being positive, and the opposite ~~occuring~~occurring for the XNOR comparator. Technically there is a tiny band where the difference between the two inputs can result in an "analog" (i.e. not logic 1, or logic 0) voltage at the output, though this may only be an issue if using the gates with analog input voltages. The outputs of the op-amp comparators are passed ~~throug~~through a diode so that only positive going voltages reach the output. Any negative going voltages (logic 0) are blocked, and the output held at 0 volts by the pull-down resistor at the output. The 1k8 resistors in line with the output, when combined with the pull-down resistors, limit the positive (logic 1) output level to around 5 volts. The LEDs indicate the output levels of the gates. As the gates share a common input structure, these LED have a complimentary relationship - if one is lit, the other is off. As such, a single LED would suffice to indicate the state of both gates. Line 29: When inserting ICs into sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay. ~~== PCB connections ==~~ {\| class="wikitable" \|+ style="text-align:left"\|PCB connections ~~! Pad ID!!Function~~ \|- \| LN\|\|to anode of XNOR LED Line 94 ⟶ 93: * Substitute 1k5 for the two 1k8 resistors if running from +/-12 volts. * PCB is 1" x 2" with two 3mm mounting holes 0.15" in from the bottom and edges. It is designed to sit above one end of a regular 2" width CGS PCB. == CC-BY-NC == Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work. == See also == Line 101 ⟶ 103: == References == * [https://web.archive.org/web/~~20170525070136fw_~~20170525070136f/http://www.cgs.synth.net:80/modules/cgs54_xor.html XOR/XNOR Logic for music synthesizers] ~~(archived)~~ by Ken Stone, 2004, with permission of the author - archived == External links == * [http://groups.yahoo.com/group/cgs_synth CGS Synth discussion group], for discussion of locating parts, modifications and corrections etc. * [http://bompiler.com/pcb/cgs54 CGS54 BOM] at BOMpiler === Suppliers === * [http://www.elby-designs.com/webtek/cgs/cgs54/cgs54_xor.html CGS54 XOR/NXOR Logic], revision 1.1, Elby Designs [[Category:CGS ~~modular~~clock/gate/trigger processors]]