V4 Link Support Tutorial

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EPICS: IOC Link Support Tutorial Oct 25 2005

Overview

The V4 link support is a replacement for V3

  1. DBF_DEVICE, DBF_INLINK, DBF_OUTLINK, and DBF_FWDLINK
  2. device DBD definition and device support
  3. driver DBD definition and driver support

The link support model is:

  • For each link, record support defines a list of interfaces it can use for communicating with link support
  • Link support defines the set of interfaces it implements

Link support has no knowledge of what record types are using it and record support has no knowledge about link support other than the interface it is using.

EPICS base defines a standard set of interfaces for link support. The intention is that the set includes enough functionality such that most link support can be implemented via just these interfaces.

The interfaces support all the functionality needed by Channel/Database access and by the generic EPICS device support supplied by asynDriver. Since asynDriver is intended as a framework for interfacing to most hardware, this means that the interfaces supplied and used by base allows support for most hardware. The support can communicate with the hardware however it wants but must implement some set of the interfaces defined by base.

This document gives a brief overview of the V4 link support model.

Database Definitions For Links

This section first reviews the DBD syntax related to link definitions. It then gives a brief description of link related definitions in dbCommon.dbd

Syntax

A record link has the syntax: 

    link(linkDirection,interface(interfaceName,...))

A link itself has the syntax: 

    link(linkDirection,choiceName,dataStructName,interface(interfaceName,...))

A record instance link definition has the syntax: 

    field = { choice(interfaceName) {structAssignmentList}}
     or
    field = { choice(interfaceName) dataStructName {structAssignmentList} }

where

linkDirection
Must be one of none,in,out,process, or inout. Compatible checks are made to match the interface with a field.
interface
This is a list of the interfaces that the record support understands or that link support implements.
choiceName
UTF-8 string that describes the choice
interfaceName
The name of an interface via which record support communicates with link support.
dataStructName
The name of a struct containing configuration information for the link support. Record support normally does not access this structure. Database configuration tools prompt the user to assign values to the structure.
choice
Selects a link with choiceName = choice

A record instance link definition selects a link as follows:

  • The linkDirections must be compatible
    • Except for inout the record link and link directions must be the same
    • If record link specifies inout the link can specify either in or out or inout
  • The interfaceName specified by the record instance must be in the record link interface list and in the link interface list.
  • The dataStructName specified in the record instance must match the dataStructName in the link definition.

Assume the following definitions: 

    record(AiRecord) {
        ...
        field(in,link(in,interface(LinkInt64,LinkFloat64)))
        ...
    }
    struct(PvMonitorLink) {
        field(pvname,string)
        field(process,boolean) // process this record when monitor occurs
        field(inheritSeverity,boolean)
    }
    struct(PvInputLink) {
        field(pvname,string)
        field(process,boolean)
        field(wait,boolean)
        field(timeout,float64)
        field(inheritSeverity,boolean)
    }
    ...
    link(in,monitorLink,PvMonitorLink,interface(
        LinkBoolean,
        LinkInt16,LinkInt32,LinkInt64,
        LinkFloat32,LinkFloat64,
        LinkString
    ))
    link(in,inputLink,PvInputLink,interface(
        LinkBoolean,
        LinkInt16,LinkInt32,LinkInt64,
        LinkFloat32,LinkFloat64,
        LinkString
    ))
    ...
    struct VME{
        field(a16,int16) 
        field(a32,int32)
        field(channel,int16)
    }
    ...
    link(in,someVmeADC,VME, interface(LinkInt32))

The first example instance definition selects a monitor link. It selects interface LinkInt32, which means the support will provide a "raw" value that the record support will convert to engineering units. 

    AiRecord aiMonitorExample = {
        ...
        in = {monitorLink(LinkInt32) 
                  PvMonitorLink {    
                       pvname = "someRecord";
                        process = true;
                       inheritSeverity = false
                  }
              }

The next example selects a channel access input link. It selects interface LinkFloat64, which means the support will provide a value in engineering units. It asks that the linked record be processed and that the link not complete until the linked record completes processing. 

    AiRecord aiInputExample = {
        ...
        in = {inputLink(LinkFloat64) 
                  PvInputLink {    
                       pvname = "someRecord";
                       process = true;
                       wait = true;
                       timeout = 1.0;
                       inheritSeverity = false
                  }
              }

The last example attaches the link to support for a VME Analog to Digital device. 

    AiRecord aiMonitorExample = {
        ...
        in = {someVmeADC(LinkInt32) VME {a16 = 0xc000; channel = 1} }

Standard Definitions For Channel/Database Access

These are the database definitions defined in dbCommon are used by the base supplied support for Channel/Database access. This support registers each choiceName in the link definitions. The complete set of data structure and link definitions are defined in "V4 DB RecordCommon".

Link support is provided for

  • monitor
  • input
  • output
  • process

For monitor, input, and output the following data types are supported:

  • primitive types
  • string
  • arrays of primitive types and strings

asynDriver link support

These are definitions that are used by the standard EPICS device support for asynDriver. Since asynDriver is a generic way of interfacing arbitrary hardware support, this is a generic way of attaching links in records to hardware.

Data Definitions

    struct(AsynLink) {
        field(portName,string)
        field(addr,int32)
        field(timeout,float64)
        field(drvPvt,string)
    }

link definitions

    link(inout,asynInt32,AsynLink,interface(LinkInt32))
    link(in,asynInt32Average,AsynLink,interface(LinkInt32))
    link(in,asynInt32Monitor,AsynLink,interface(LinkInt32))
    link(inout,asynFloat64,AsynLink,interface(LinkFloat64))
    link(in,asynFloat64Average,AsynLink,interface(LinkFloat64))
    link(in,asynFloat64Monitor,AsynLink,interface(LinkFloat64))
    link(in,asynDigitalMonitor,AsynLink,interface(AsynDigital))
    link(inout,asynDigital,AsynLink,interface(AsynDigital)))
    link(inout,asynInt32Array,AsynLink,interface(LinkArrayInt32)))
    link(inout,asynFloat64Array,AsynLink,interface(LinkArrayFloat64)))
    link(inout,asynOctet,AsynLink,interface(AsynOctet)))

This is the set of definitions for the standard EPICS device support implemented by asynDriver. Although, at least for the first few V4 releases, asynDriver will not be part of base, they are shown here because the LinkInt32,... interface definitions are defined as part of base. Again it is expected that almost all hardware support be created by implementing a combination of these link interface definitions or the interfaces implemented by Channel/Database access link support. ---

Link Support

Overview

An arbitrary number of Link Support implementations can exist. An implementation can be either soft support or support that communicates with hardware.

Other link support can also be supplied. Whenever possible support should implement the standard interfaces supplied by EPICS base, since these are the interfaces that the record types supplied with base know how to use.

Soft support should try to implement the same interfaces implemented by the Channel/Database access support supplied with base. The support can, of course, define data structures for it's own use. Implementing these interfaces means that the support will work for the same set of record links as the Channel/Database access support.

Hardware support should, if possible, also implement the same interfaces implemented by the Channel/Database access support. Message based support may also have to implement the AsynOctet interface but this interface is only used by a few record types.

This section describes the interfaces implemented by all link support and then the standard interfaces used by the records supplied with base.

The syntax is similar to Java syntax since it is more concise than C++. It can easily be translated to C++ syntax. For example the definition: 

    interface LinkInt16 {
        LinkResult get(Callback callback,Int16 data);
        LinkResult put(Callback callback,Int16 data);
        void getBounds(<type> low, <type> high);
    }

In C++ would be: 

    class LinkInt16 {
    public:
        virtual LinkResult get(Callback &callback, int16_t *data) = 0;
        virtual LinkResult put(Callback &callback, int16_t data) = 0;
    };


Link and MonitorLink

NOTE: arguments and return types ignored for now 

    interface Link {
        void report(int16 level);
        void cancel();
        void destroy();
        void initialize();
        void connect();
        void disconnect();
    }

    interface MonitorLink {
        void addMonitor(Callback callback);
        void removeMonitor();
    }

Link is an interface that must be implemented by every link support. An instance of this is connected to each DbfLink field. The Link methods are:

  • report - report
  • cancel - Cancel any outstanding I/O
  • destroy - This is called if the field is being changed after initialization or if the record is being removed.
  • initialize - Called to initialize a link.
  • connect - Called to connect. Note that this is different than initilization.
  • disconnect - disconnect.

MonitorLink is an interface that is implemented by link support that supports monitors. An example is support for hardware interrupts. The MonitorLink methods are:

  • addMonitor - Add a monitor callback
  • removeMonitor - Remove monitor

Normally record support does not need to call any of the Link or MonitorLink methods since database access does this automatically. For example if a link field is modified via a channel access put, database access will call destroy before modifying the link and initialize and connect after the link is modified.

Note that neither Link or MonitorLink is listed in the link database definition since they are generic.

Definitions that apply to Process,Monitor,Input,and Output support

These are the definitions used by the Channel/Database access support supplied with base. It is also used by the standard EPICS support from asyn. 

    enum LinkResult {
        linkNoop,           // Nothing was done, e.g. link is null link
        linkDone,           // field was modified. No wait is necessary
        linkWait,           // waiting. can do additional procsssing
    }

    interface Callback {
        void done();
        void failure();
    }

Process Link Support

    interface LinkProcess {
        LinkResult process(Callback callback);
    }

Octet Support

    interface LinkOctet {
        LinkResult get(Callback callback, Octet data);
        LinkResult put(Callback callback, octet data);
    }
    interface LinkArrayOctet {
        LinkResult get(Callback callback, octet[] data);
        LinkResult put(Callback callback, octet[] data);
    }

GENERIC QUESTION The data is passed as primitive or arrays of primitive types. Should Dbf interfaces be used? Needs thought.


asynOctet Support

This still needs more work. It attempts to reproduce the functionality of V3 asynDriver. 

    interface AsynOctet {
        LinkResult write(Callback callback,
          octet[] data, int32 numchars, Int32 nbytesTransfered);
        LinkResult writeRaw(Callback callback,
          octet[] data, int32 numchars, Int32 nbytesTransfered);
        LinkResult read(Callback callback,
          octet[] data, Int32 nbytesTransfered);
        LinkResult readRaw(Callback callback,
          octet[] data, Int32 nbytesTransfered);
        void flush();
        void setInputEos(octet[] eos);
        void getInputEos(octet[] eos);
        void setOutputEos(octet[] eos);
        void getOutputEos(octet[] eos);
    }

    enum interruptReason {
        interruptOnZeroToOne, interruptOnOneToZero, interruptOnBoth
    }
    struct asynDigitalInterrupt {
        octet[] mask;
        int32 addr;
        Callback callback
    }

    interface AsynDigital {
         LinkResult write(Callback callback,octet[] value, octet[] mask);
         LinkResult read(Callback callback,octet[] value, octet[] mask);
         void setInterrupt(octet[] mask, interruptReason reason);
         void clearInterrupt(octet[] mask);
         void getInterrupt(octet[] mask, interruptReason reason);
         void registerInterruptUser(interruptCallbackUInt32Digital callback,
             octet[] mask);
         void cancelInterruptUser();
    }

Boolean Support

    interface LinkBoolean {
        LinkResult get(Callback callback,Boolean data);
        LinkResult put(Callback callback,boolean data);
    }
    interface LinkArrayBoolean {
        LinkResult get(Callback callback,boolean[] data);
        LinkResult put(Callback callback,boolean[] data);
    }


The data source must be a boolean or a string that contains a valid boolean value.

Integer Support

Support is available. for int16, int32, and int64. 

    interface LinkInt16 {
        LinkResult get(Callback callback,Int16 data);
        LinkResult put(Callback callback,Int16 data);
        void getBounds(<type> low, <type> high);
    }
    interface LinkArrayInt16 {
        LinkResult get(Callback callback,int16[] data);
        LinkResult put(Callback callback,int16[] data);
    }
    interface LinkInt32 {
        LinkResult get(Callback callback,Int32 data);
        LinkResult put(Callback callback,Int32 data);
        void getBounds(<type> low, <type> high);
    }
    interface LinkArrayInt32 {
        LinkResult get(Callback callback,int32[] data);
        LinkResult put(Callback callback,int32[] data);
    }
    interface LinkInt64 {
        LinkResult get(Callback callback,Int64 data);
        LinkResult put(Callback callback,Int64 data);
        void getBounds(<type> low, <type> high);
    }
    interface LinkArrayInt64 {
        LinkResult get(Callback callback,int64[] data);
        LinkResult put(Callback callback,int64[] data);
    }

    enum interruptReason {
        interruptOnZeroToOne, interruptOnOneToZero, interruptOnBoth
    }

Float Support

Support is available. for float32 and float64. 

    interface LinkFloat32 {
        LinkResult get(Callback callback,Float32 data);
        LinkResult put(Callback callback,float32 data);
    }
    interface LinkArrayFloat32 {
        LinkResult get(Callback callback,float32[] data);
        LinkResult put(Callback callback,float32[] data);
    }
    interface LinkFloat64 {
        LinkResult get(Callback callback,Float64 data);
        LinkResult put(Callback callback,float64 data);
    }
    interface LinkArrayFloat64 {
        LinkResult get(Callback callback,float64[] data);
        LinkResult put(Callback callback,float64[] data);
    }

String Support

    interface LinkString {
        LinkResult get(Callback callback,string data);
        LinkResult put(Callback callback,string data);
    }
    interface LinkArrayString {
        LinkResult get(Callback callback,string[] data);
        LinkResult put(Callback callback,string[] data);
    }


The data source must be a string.

Example - VME ADC support

Assume the VME and link definitions given in the examples at the beginning of this document.

The following is presented with Java syntax. It assumes that there is VME support of the form

public class vme { 

   static public short getShort(long addr);
   ...

}

The link support would be something like: 

    class AdcSupport implements Link LinkInt32 {
    AdcSupport(VME fromFactory)
    {
        addr = fromFactory;
        connected = false;
        //  from the VME initailze vmeaddr and channel
        // details left to your imagination
    }

    // Link methods
    void report(int16_t level)
    {
        printf("AdcSupport a16 %x channel %d\n",a16,channel);
    }
    void cancel() {} // nothing to do
    void destroy() {} //nothing to do
    void initialize()
    {
        // make sure we can access a16
    }
    void connect() {connected = true}
    void disconnect() {connected = false}
    
    // LinkInt32 methods
    LinkResult get(Callback callback, Int data)
    {
        data = (Int)VME.getShort(vmeaddr);
        return linkDone;
    }
    LinkResult put(Callback callback, Int data)
    {
        throw CantWriteAdc;
    }
    void getBounds(Int low, Int high) 
    {
        // assume 16 bit unipolor adc
        low = 0; high = 0xffff;
    }
    private:
        VME addr;
        int vmeaddr;
        int channel;
        boolean connected;
    }
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