ESA PSS-05-10 Issue 1 Revision 1 (March 1995)
as shown by the structure chart. Two possible assembly sequences are
shown. The sequence starting with M1 is 'top-down' and the sequence
starting with M2 is 'bottom-up'. Figure 2.6.1.2B shows that data flows from
M2 to M3 under the control of M1.
Each sequence in Figure 2.6.1.2A requires two test modules. The
top-down sequence requires the two stub modules S2 and S3 to simulate
M2 and M3. The bottom-up sequence requires the drivers D2 and D3 to
simulate M1, because each driver simulates a different interface. If M1, M2
and M3 were tested individually before assembly, four drivers and stubs
would be required. The incremental approach only requires two.
The rules of incremental assembly argue for top-down assembly
instead of bottom-up because the top-down sequence introduces the:
• modules one-by-one;
• producer modules before consumer modules (i.e. M1 before M2 before
M3).