Examining a PCB and drawing a Schematic Diagram : Reverse Engineering a 6502 CPU card

In this video, which is a continuation from the retro 6502 repeater project, we are reverse engineering our 1980's PCB card to schematic.
This is the first step - we are making a rough pencil and paper diagram and then make corrections as we go. By tracing PCB tracks and looking at data sheets, we can begin to build up a full schematic of the computer board.
This will lead to re-drawing the schematic using Ki-Cad Schematic Editor in the next video. So look out for that too.
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Leave a comment! Do you like long format videos or would you rather see smaller 10-20 minute ones. What kind of content would you like to see? More digital, more analog, CPU/MCU? Let me know.

[@g4ekm]

looking forward to the next video.

[@tdrelectronics766]

The 2k7 resistor is to stop the nand gate driving the DLatch signal inputs from floating if the two diodes anodes are logic 0
Also the way I approach reverse engineering is to use a database and spreadsheet. List all the components on the spreadsheet ie R1 R2 C1 IC1 etc
then put the pin numbers in ue pin1 and pin2 for R1 (decide what is pin1 and pin2 for all discretes ie pin 1 is left or top and pin 2 is right or bottom diodes is obviously cathode and anode the ic are where the database comes in for example the 6502 has 40 pins you could search for 6502 in your database and then copy the result to the spreadsheet in the form of ic name 6502 in this case the pin number pin 1 pin 2 etc the pin name D1 D2 ADDR1 ADDR2 etc then what type of pin it is ie all the data pins just put DATA all the address pin would be ADDR when the spreadsheet is complete you can use the filter function to just filter all the DATA pins and this should highlight all the pins that are DATA ie the processor the eprom and ram
once you see what the pins are you can quickly check with a multimeter what pins are connected between these devices once you've proved a connection exist you can give the net connecting these connections a net name such as D1 D2 etc this quickly eliminates a lot of the nets. power nets are quickly eliminated in this way obviously at the end you will be left with a few component pins that are not connected for example the 3 address lines going to the 72ls138.
The way that you approached this card is fine for such a simple circuit works fine but for modern electronics where the components are surface mount and the pcb layer count is more than 2 my way would be quicker I hope this makes sense to you but overall, the video was enjoyable

[@NEEC1]

HI there. The purpose of the 2k7 is a pull down resistor forming part of a logic NOR gate. That NOR enables the output port D-latches, its basically its write signal. Without the 2k7, the input of the NAND would remain high when the diodes are at zero. When i designed this back in 1986, i could have used more logic chips to get the same result ie. actual NOR gates, but wanted to keep the chip count at a minimum. So i opted for a simple diode logic NOR here instead. Two diodes and a resistor eliminated a whole chip. Hope that helps. Thanks for your interest !