part-rnd - design

part-rnd keeps two type of databases:

• rnd parts: the user's own part names for virtual parts
• sourced parts: parts from various sources (different vendors, webshops, the user's local stock/inventory)

The schematics contain rnd part names. An rnd part is specific in the sense of "what kind of part can be used here" but generic in the sense of "where the actual part is sourced from". An rnd part name normally looks something like "R-4.7k-1206". The rnd part ID is an attribute of the sch-rnd (abstract) component, typically coming from a symbol attribute or from a devmap.

At the end of all annotations and exports, there is a BoM file that is used for two purposes: purchasing parts and assembling to board. This BoM file would still contain the rnd part names which are not useful for either purpose. This is where part-rnd joins the workflow and translates rnd into sourced part numbers. This translation is possible because:

• rnd parts refer to a bunch of different sourced parts, so part-rnd can look up what actual products can be source from where for the given part in the input BoM
• part-rnd has plugins to contact external sources, e.g. webshops and other databases, to verify the price and availability of sourced parts
• the same plugins allows the user to do searches in those external sourced part databases to make the references from rnd parts

The translation, the extra indirection, the fact that the design contains rnd part names and not hardwired part numbers of a specific shop solves multiple problems, because R-4.7k-1206 in the rnd database is a generic concept of a 4.7k ohm resistor in 1206 package. Most notably:

• a specific webshop may sell multiple resistors from different manufacturers matching the specification; whichever is available on the week of purchase may be random
• if multiple options are available, the price may differ
• a board designed 2 years ago still has the same resistor today, but the shop-specific part number may have changed over time
• there can be different shops used for sourcing, e.g. one for prototypes, another for larger batches; each will sell the same 4.7k resistors but under different part numbers

Thus hardwiring shop part numbers in the design is suboptimal because it breaks as shops change their inventory or as you change which shop you buy from.

The part-rnd indirection solves some of these by letting the user make cross referencing between the generic rnd and the "state today" sourced databases separately from the design. Once the cross reference is done, it will work for multiple designs automatically and will need only minimal maintenance as shop inventories change in time. For example whenever an update needs to be done to the rnd 4.7k 1206 resistor, all matching parts from all potential shops can be linked to it. It's basically done by doing a search (in external databases) for something like "resistor 4.7k 1206"; this will result in a list of hits; then the user simply marks all items on this list that can be used for the given generic rnd part.

Once cross references are available, part-rnd can now or in the future go and figure which parts are available at the moment and which are the cheapest at the quantity needed for the given batch.

The rnd database, including part names, is designed by the user. The actual part names and the specific concepts they represent are all up to the user.

Example

A hypothetical example board has 4.7k resistors and 2n7002 fets and a 8 pin connector.

• invent rnd part numbers for these parts: R-4.7k-1206, 2n7002-sot23, conn-100mil-8x1
• use a text editor to create a part-rnd description of these three, as three files in the part-rnd database in subdirectory rnd/
• draw the schematics
• add symbol attributes (TODO) so that all resistor, 2n7002 and connector symbols refer to the corresponding rnd
• forward annotate, design the board in pcb-rnd
• export gerbers, xy and BoM
• load the BoM with part-rnd
• do the cross referencing in part-rnd
  • in the BoM table part-rnd notices it can not find sourced part number for rnd part R-4.7k-1206
  • do a search for "resistor 4.7k 1206" in external databases; this imports all resulting parts in the sourced database (which acts as a local cache)
  • go through the result list and mark the ones that look good in general
  • tell part-rnd to bind the marked sourced parts to R-4.7k-1206 (this will modify the file in rnd/R-4.7k-1206 to include the selected references to shop-specific part numbers)
  • repeat this process for rnd part 2n7002-sot23
  • repeat this process for rnd part conn-100mil-8x1
• now all rnd parts in the BoM in part-rnd has at least one (but preferably more) shop part number associated from the sourced database
• run the translator; it will pick the best sourced part for each rnd part
• revise the result, make manual changes of sourced part choice if needed
• export a BoM with the sourced part numbers (shop part numbers)

What the best choice for a part is depends on many factor: price at the given quantity, availability, other constraints like "using jlcpcb's assembly service works best when parts are sourced from lcsc". These heuristics are scripted and can be reconfigured or new scripts can be written by the user.