Page 243 - The Art of Designing Embedded Systems

230 THE ART OF DESIGNING EMBEDDED SYSTEMS

First, note that each of the three BOM types (Le., dash numbers) is
listed at the beginning of the parts list. A column is assigned to each dash
number; the quantities needed for a particular dash number are in this col-
umn. That is, there is a “quantity” column for each BOM type.
The first three entries, one per dash number, simply itemize what
each dash number is. The quantity must be zero.
Each dash number column contains all quantity information to make
that particular variation of the BOM.
Next, notice that drawing “#1892” is called out with a quantity of 0.
Drawing #1892 shows how the parts are stuffed into the board, and is
essential to production. However, it cannot call parts that must be bought,
so it always has a quantity of 0.
The schematic and test procedure are listed, even though these are
not really needed to build the unit. This is how all non-production engi-
neering documents are linked into the system. All schematics, test proce-
dures, and other engineering documentation that we want to preserve
should be listed, but the quantity column should show 0. Notice also that a
drawing number is assigned even to the test procedure. This insures that
the test procedure is linked into the system and maintained properly.
The first column is the “item number.” One number is assigned to
each part, starting from 1 and working up. This is used where a mechani-
cal drawing points out an item; in this case the item number would be in a
circle, with an arrow pointing to the part on the drawing. It forms a cross
reference between the pictorial stuffing drawing and the parts list. In
most cases most item numbers will not have a corresponding circle on the
drawing.
All jumpers that are inserted in the board are listed along with how
they should be inserted (by the reference designator). This is the only doc-
umentation about board jumpering we need to generate.
Note that no modifications to the PCBs are listed. PC board modifi-
cations are to be listed on a separate “Mod” drawing, which is also refer-
enced with a quantity of zero on the BOM.

ROMs and PALS
Every ROM and PAL used in a unit will be called out by two entries
in the parts list columns of the PC board BOM. The first entry calls out the
device part number (like GAL22V10) and associated data so purchasing
can buy the part. The second entry, which must follow right after the first,
calls out a ROM or PAL BOM.