<b>Conversion,</b> converting legacy system data into SAP data. Meaning that it is mainly a one shot deal, for example bringing your customer master to SAP from the legacy system, converting the fields used in legacy to the fields and values used in SAP.
<b>Interface,</b> a program or group of programs that handle the passing of data from one system to another. These programs work together to export the data from one system and import into a target system, maybe these programs run once a day, maybe twice, may every hour throughout the day. It is a good way to keep the data in two or more systems in sync with each other.
Interface programs are also data extract program. Data is extracted from SAP into a flat file and transferred to other external systems like UNIX, mainframe.
In data extracts, the file layout is designed and this layout should be in synch with other external systems.
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CONVERSIONS are done only once. ie ALL the data from legacy system is transferred to SAP system once.Generally we do this transfer for MASTER DATA where we have to transfer all the master data at a time initially.
INTERFACES are periodic transfer. it can be done weekly,monthly or yearly depending upon the need .we do this for transactional data
<b>Interface:</b> it is a program that performs communication with External system from SAP.
There are two types of interfaces:
1. Inbound Interface: External system sends the data to SAP
2. Outbound Interface: SAP sends the data to External system.
Let's take a scenario:
There are two Systems SAP ERP and Seibel CRM
Scenario1: Whenever an Order is created in Seibel CRM system it has to be transferred to SAP where the Order confirmation, Delivery and Billing will be performed. This can be enabled by using IDOC as inbound interface.
Scenario2: Whenever a New customer is Created in SAP ERP this customer data has to be transferred to Seibel CRM using Outbound IDOC / RFC.
Basically interfaces are implemented using RFC, BAPI, ALE/IDOC.
Migration of data from Legacy system before GO Live (One time data transfer).
So Conversions are always INBOUND interfaces.
Conversions are performed using Batch input(BDC), LSMW and BAPI.
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In terms of SAP
Conversion, converting legacy system data into SAP data.example using the methods LSMW, BDC
(In this process earlier running apllication data to sap data.)
Interface : if u r aware of Remote funtion calls (type of interface)
if u go through java RMI (interfaces) ,
In client server arch we use interfaces ie communication between known objects between clint &server .
(taking example the jvm with java complier & abap )