1.11 - User Guide for EPICS Users

Overview

AIDA-PVA is a system for providing programmatic access to SLC Control System devices, data and services. Since its introduction in 2002 it has been used in other laboratories in the US and around the world. It provides a naming convention to address all devices and their attributes, a way of specifying arguments for remote procedure calls and obtaining results in any format, including rich formats with including metadata. Finally, it provides security, logging and reliability to allow it to be used in a variety of applications from logging, and monitoring, to modeling and control.

AIDA-PVA has recently been upgraded to use EPICS-7's PVAccess mechanism to transport requests to the Data Providers.

Components

AIDA-PVA uses EPICS as shown below.

EPICS in AIDA-PVA

Some basics

• AIDA-PVA uses PVAccess but only the RPC protocol. So other protocols won't work with AIDA-PVA.
• AIDA-PVA uses Normative Types to encapsulate both data and requests. The types used are:
  • NTTable - to return tabular data from RPC requests
  • NTScalar - to return scalar data from RPC requests
  • NTScalarArray - to return scalar arrays from RPC requests
  • NTUri - to create requests to AIDA-PVA Channel Providers

What works the same way

• Environment variables - all EPICS environment variables work the same way as they do in EPICS. The most important are:
  • EPICS_PVA_ADDR_LIST - this is the list of addresses to search for Channel Providers. Set it to mccdev.slac.stanford.edu for testing against the development environment.
  • EPICS_PVA_AUTO_ADDR_LIST - set this to FALSE so that EPICS won't try to automatically create your address list
• The channel addressing scheme does not change. All of the implemented Channel Providers are fully compatible with the channel addressing scheme.

What's a little bit different

• Disambiguation - some channels in the SLAC namespace are supported by more than one Channel Provider. So to allow clients to have coherent and reliable results disambiguation is achieved by prefixing the channel name with the service provider short code and two colons. e.g. SLC::KLYS:LI31:31:PDES

  Warning

  In the case of an incorrectly specified channel, a provider will notify the client that it must add the appropriate prefix before resubmitting the request.

• EPICS RPC does not have the concept of setting a value, normally that is reserved for put. In AIDA-PVA we've added the "set-value" semantic to RPC by using the VALUE argument. In AIDA-PVA the VALUE argument is a reserved word and any request that has the VALUE argument is treated as a setter.

  import org.epics.pvaccess.ClientFactory;
  import org.epics.pvaccess.client.rpc.RPCClientImpl;
  import org.epics.pvaccess.server.rpc.RPCRequestException;
  import org.epics.pvdata.pv.*;
   
  public class AidaPvaRunner {
      public static void main(String[] args) {
          RPCClientImpl client = new RPCClientImpl("CHANNEL");
          
          // ...
        
          // Only do this if the request is to set a value
          PVStructure query = request.getStructureField("query");
          ((PVString) (query.getSubField("VALUE"))).put("A Value");
          
          // Execute the query with a timeout of 3 seconds
          try {
              PVStructure result = client.request(request, 3.0);
   
              // Use result ...
          } catch (RPCRequestException e) {
              // Do something with error
          } finally {
            client.destroy();
          }
      }
  }

• EPICS RPC does not have the concept of returning different types from the same channel. Normally a channel is associated wit a static return type. In AIDA-PVA the TYPE argument is a reserved word and any request that has the TYPE argument is specifying the return type of the request. Additionally, some Channel Providers implement TABLE_TYPE, which is also a reserved word, to specify the desired return-type of a single column table request.
  

  // Set the request query return type
  PVStructure query = request.getStructureField("query");
  ((PVString) (query.getSubField("TYPE"))).put("FLOAT");
   
  // Execute the query with a timeout of 3 seconds
  try {
      PVStructure result = client.request(request, 3.0);
   
      // Use result ...
  } catch (RPCRequestException e) {
      // Do something with error
  }