Wiring diagrams are more detailed than functional block diagrams. Wiring diagrams show all the cables and wiring harnesses between subassemblies in the system.

For example, consider the setup of a desktop computer:

A wiring diagram does not show details of the subassemblies, or of the cables themselves. The diagram's purpose is to see all the physical interconnections between subassemblies within the system. Further details can be added to the wiring diagram like item numbers for cables, so the full technical data package includes functional requirements like wiring pinouts.

NOTE
Notice that the Speaker and Microphone share a common audio jack on the computer. This demonstrates a wye-cable or wiring-harness, and how it connects multiple ports.

ASME Y14.44 standardises the reference letter designations for different components like 'A' for assemblies, and 'K' for relays. This standard also suggests how to label and reference systems with multiple units and assemblies.