ERP vs MES, in plain terms.

What an ERP is for

An ERP runs the business. Orders, purchasing, inventory, costing, invoicing. Its natural unit is the order.

It is the system that answers: what did we commit to, what did it cost us, what are we owed and what do we owe. Finance usually buys it, and finance usually keeps it.

What an MES is for

An MES runs the floor. What is on which machine right now, who is running it, at what rate, what stopped and why. Its natural unit is the job or the operation.

It is the system that answers: what is actually happening, and what actually happened. The floor asks for it, usually after the ERP turns out not to hold that detail.

Where the line blurs

Scheduling. Both categories claim it, and they approach it from opposite ends. An ERP schedules backward from what was promised. An MES schedules forward from what the floor can actually do.

When the two disagree, and they do, somebody reconciles it by hand. That person is usually the one who knows the most and has the least time.

Why shops end up with both

Rarely by plan. The usual sequence is an ERP first, because the business needs to invoice, followed some years later by an MES, because the ERP cannot describe a job in enough detail to be useful on the floor. Older shops sometimes run it the other way round.

Neither vendor is being dishonest about this. The two systems were built for different questions.

The spreadsheet in the middle

Almost every floor running both has one. It holds the real schedule, or the real yields, or the exceptions neither system can express. It is maintained by one person and it is usually right.

If your most accurate system is a spreadsheet somebody maintains by hand, the problem is not which platform you bought.

So which do you need?

A rough test, and it is not the one either vendor will give you:

• If you cannot say what you committed to and what it cost, that is an ERP gap.
• If you cannot say what is running now and why it stopped, that is an MES gap.
• If you can answer both, but somebody retypes between them every week, that is neither. A third system will not fix it.

Where we sit

AMI is not an ERP and it is not an MES. We are not asking you to add a third one. We read what the systems you already run produce, so the work between them stops being manual.

If the third case above is the one that sounds like your floor, that is the conversation we want.