Sooner or later, you are going to come across PCBs with components that do not have their reference designators printed on the silkscreen layer, either due to congestion of space or by design. What then?
Well, you can do the following steps:
1. Take note of component designators that are available on the component and solder sides' silkscreen layers of the PCB. Note down the largest number for each of the group designators (e.g. R123, C99, U68, etc.).
2. Determine the arrangement or layout of the components, paying attention to the way the reference designators run on the PCB, horizontally or vertically, from left to right or vice versa. Then follow the flow by adopting either a row or column numbering pattern accordingly.
3. Make a photocopy scan of the PCB, grayscale and inverse it, and then segment it into grids as shown below. Start numbering the unmarked resistors based on the grid reference: R1A, R1B, R1C, etc.
The above example uses column numbering pattern due to the layout of the components which makes it suitable for this scheme. Sometimes the components' layout maybe haphazard or random, in which case you can still produce a grayscale-inverse artwork of the PCB, print it out and manually write on the hardcopy. This is the fastest way to assign designators, but is quite messy and error prone.
A better way is to wait till you complete drawing the Visio layout diagram of the PCB before assigning the designators (what this book teaches). This will save you a lot of trouble in case you miscount or miss-count the components and need to white-out or erase the errors and re-number the designators on paper.
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