ECOs and version numbers

It is important to be able to track different versions of hardware, in exactly the same way as we have to track different versions of software. To this end we use drawing numbers, revision levels, ECOs and controlled documents. The method described here might seem rather rigorous, especially for a small project, but once it is in place it reduces the scope for errors considerably, and costs little in effort to run.

At AirBorn electronics, we use a drawing number (e.g. AB9407111) on our circuit diagrams that refers to an identically numbered Printed Circuit Board and an identically numbered parts list - tieing the group together. Thus every time a new version of a PCB is made, its matching files form a unique set... Actually, by virtue of a special "component" the number is carried through to the PCB and parts list automatically, so it takes some amount of effort if we ever want to remove it from the other documents!

Circuit diagrams have a revision number (e.g. "V1.02a") that is basically a climbing record of changes to the document. This number is independent of the drawing number, as sometimes there will be changes to a schematic that can be reworked on existing PCBs. An example could be a different resistor value - a new schematic diagram might be required, but existing PCB laminates could be used with just a simple modification. The drawing number would remain the same, as a new PCB has not been issued - but the schematic revision level would climb, to reflect the new issue of diagram.

It is ofcourse also possible (although more rare) that a printed circuit board would be reissued with no modification to the schematic. This would normally be because of some mechanical change (e.g. moving a component or mounting hole) or cosmetic (e.g. labelling) change of the PCB. In this case the PCB number would change on all documents - but the schematic revision number could remain the same, indicating that this was, in fact, exactly the same circuit design.

When a modification is required to equipment that has been shipped or is otherwise outside of the immediate control of the electronic designers, an Engineering Change Order (ECO) is issued. An ECO is an instruction to implement an engineering change, and must specify the equipment to be altered (PCB number and revision level), the modification, and its applicability.

Applicability of ECOs determines which units will be modified - some common definitions might be:

• Mandatory on all current and future production
• Mandatory on all units held in stock
• Optional on units already in the field (subject to the occurrence of symptoms)
• Mandatory on all current production only
• Not applicable to future production (future units must comply with xxx)

The ECO will usually include a statement of why the ECO was required, or what production or design fault the ECO remedies. Often modifications are made to equipment to accommodate a change in some outside factor - e.g. a shift in specification of a certain part used on the board, or a problem with an interfacing device. The number of ECOs is not a reflection on the quality of the design - often it is easier to alter a PCB that is in regular production to work around a fault in an interfacing device, and so an ECO might be issued to this effect - even though, strictly speaking, there was no problem in the PCB being modified.

The original issue documentation files are "Controlled documents". A record is kept of the location of the documents (in practice this usually means the files themselves are kept in a safe place!), and whenever a modification occurs to the equipment, the controlled documents are updated. In this way, the controlled document always represents the known latest version of the schematic. Part of the documentation will be a record of past revisions.

Uncontrolled documents are copies of controlled documents at an instant in time, and may be out of date if modifications have occurred since that time.

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