ERP systems and sales points

No they don't have to be connected all day. In fact for robustness, I'd opt not to. This allows the ERP system to sit at a central site and the points of sale to work independently as required where they are needed. If there is a communication problem the systems are not adversely affected - there is nothing more frustrating for a customer than a point of sale that locks up because of a problem elsewhere in the corporate system.

In this loosely coupled situation, transactions are batched upand transmitted to the ERP system. How this batching up will happen depends on the Point of Sale system installed and how it is configured. I know of some where the point of sale tills are PC based and act individually and others where all the tills are linked to a server on the network to batch the transactions. The time interval will be business dependant: it may be every hour or overnight.

The physical link may be dial up, leased line or secure private network.

The protocol between the Point of Sale System and the EPR system will depend on what software is installed. It could be XML, EDI, as a comma separated file or a proprietary format that is recognised by the suite they are part of.