V4 Link Support Tutorial
V4 Link Support Tutorial - November 03 2005
Overview
The V4 link/struct support is a replacement for V3
- DBF_DEVICE, DBF_INLINK, DBF_OUTLINK, and DBF_FWDLINK
- device DBD definition and device support
- driver DBD definition and driver support
Each link and struct field in a record can have associated support.
- record support communicates with support via interfaces
- link or struct support implements interfaces
- link or struct support registers the interfaces it implements
- record support looks for interfaces it uses
- it uses only the interfaces it finds
- if no interfaces are found it reports an error
Support has no knowledge of what record types are using it. Struct support may have knowledge of the struct field but not of the record type. recordSupport has no knowledge about support other than the interface it is using.
For the rest of this document recordSupport means record support and support, unless qualified, means link or struct support.
EPICS base defines a standard set of interfaces for support, which includes enough functionality such that most link support can be implemented via just these interfaces.
Base provides struct support for the following:
- RecordCommon - used by all record types in base
- LinearConvert - can be used by any analog type record
- others TBD
Base provides channel/database access link support for primitive, string, and array fields. The link interfaces provided by base are also used 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 anyway it wants but must implement some set of the interfaces defined by base.
This document gives a brief overview of the V4 link/struct support model.
Database Definitions For links and structures
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:
field(name,link)
A record struct has the syntax
field(name,struct(structName))
Support for a struct or link has the syntax
support(choiceName) // no SupportStruct
support(choiceName,SupportStruct)
A record instance link has the syntax
field = support(choiceName) SupportStruct{structAssignmentList}
A record instance struct has the the syntax
field = StructName{structAssignmentList} // no support
field = support(choiceName)
StructName{structAssignmentList} // support has no SupportStruct
field = support(choiceName) SupportStruct{structAssignmentList}
StructName{structAssignmentList}
- structName
- The name of a struct
- choiceName
- UTF-8 string that describes the choice. Each support must have a unique name.
- StructName
- The same name that appears in the field definition of the record.
- SupportStruct
- The name of a struct containing configuration information for the support. recordSupport normally does not access this structure. Database configuration tools prompt the user to assign values to the structure.
Assume the following definitions:
record(AiRecord) {
...
field(in,link)
...
}
...
struct(MonitorLink) {
field(pvname,string)
field(process,boolean) // process this record when monitor occurs
field(inheritSeverity,boolean)
}
struct(InputLink) {
field(pvname,string)
field(process,boolean)
field(wait,boolean)
field(timeout,float64)
field(inheritSeverity,boolean)
}
...
support(monitorLink,MonitorLink)
support(inputLink,InputLink)
struct VME{
field(a16,int16)
field(a32,int32)
field(channel,int16)
}
...
support(someVmeADC,VME)
The first example instance definition selects a monitor link.
AiRecord aiMonitorExample = {
...
in = {
support(monitorLink) MonitorLink {
pvname = "someRecord";
process = true;
inheritSeverity = false
}
}
The next example selects a channel access input link.
AiRecord aiInputExample = {
...
in = {
support(inputLink) InputLink {
pvname = "someRecord";
process = true;
wait = true;
timeout = 1.0;
inheritSeverity = false
}
}
The last example selects support for a VME Analog to Digital device.
AiRecord aiMonitorExample = {
...
in = {support(someVmeADC) VME {a16 = 0xc000; channel = 1} }
Standard Definitions For Channel/Database Access
menuStructBase.dbd defines definitions for Channel/Database Access Links. The base supplied support registers each choiceName in the support definitions. The complete set of data structure and support definitions are defined in "V4 DB RecordCommon".
Link support is provided for
- process
- monitor
- input
- output
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)
}
support definitions
support(asynInt32,AsynLink)
support(asynInt32Average,AsynLink)
support(asynInt32Monitor,AsynLink)
support(asynFloat64,AsynLink)
support(asynFloat64Average,AsynLink)
support(asynFloat64Monitor,AsynLink)
support(asynDigitalMonitor,AsynLink)
support(asynDigital,AsynLink)
support(asynInt32Array,AsynLink)
support(asynFloat64Array,AsynLink)
support(asynOctet,AsynLink)
This is the set of definitions for the standard EPICS device support implemented by asynDriver. At least for the first few V4 releases, asynDriver will not be part of base. ---
interfaces for link and struct support
Support can be provided for link and struct fields. An arbitrary number of Support implementations can exist. An implementation can be either soft support or support that communicates with hardware.
The syntax for interface definitions 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);
boolean getBounds(Int16 low, Int16 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;
virtual bool getBounds(int16_t &low, int16_t &high) = 0;
};
NOTE: Int16 becomes int16_t &data and int16 data becomes int16_t data.
Support and SupportMonitor are interfaces that are common to the support for both link and struct fields.
NOTE: arguments and return types ignored for now
interface Support {
void report(int16 level);
void destroy();
void initialize(int16 pass);
void connect();
void disconnect();
void cancel();
}
interface SupportMonitor {
void addMonitor(Callback callback);
void removeMonitor();
}
Support is an interface that must be implemented by every support module. SupportMonitor is an interface that is implemented by support that supports monitors. An example is support for hardware interrupts.
Normally recordSupport does not need to call any of the Support or SupportMonitor 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. ---
interfaces for struct support
support for RecordCommon
This implements an interface, not yet defined, that is similar to the interface record support implements.
support for LinearConvert
This implements the interface
interface SupportLinearConvert{
convert(int32 raw,Float64 value) // from raw to eng
convert(float64 value,Int32 raw) // from eng to raw
}
In addition it provides a factory
interface FactoryLinearConvert{
SupportLinearConvert(string choiceName,
DbfStruct linearConvert,DbfLink inlink)
}
support for simulation
This needs to be defined.
support for breakpoint tables
This needs to be defined.
---
interfaces for link Support
EPICS base defines a number of standard interfaces that are used by the record types supplied with epics base. These interfaces are appropriate for the soft support supplied with EPICS base and should also be sufficient for a wide variety of hardware support. In particular they sufficient for communication with asynDriver.
The standard interfaces use the following definitions:
enum LinkResult {
linkNoop, // Nothing was done, e.g. link is null link
linkNoop, // field was modified. No wait is necessary
linkWait, // waiting for completion
}
interface Callback {
void done();
void failure();
}
The complete set of interfaces are described in V4 Design: Runtime Interfaces. Some examples are:
interface LinkProcess {
LinkResult process(Callback callback);
}
interface LinkInt16 {
LinkResult get(Callback callback,Int16 data);
LinkResult put(Callback callback,int16 data);
boolean getBounds(Int16 low, Int16 high);
}
LinkProcess is the interface for process link fields and LinkInt16 is the interface for int16 fields.
Example - VME ADC support
Assume the VME and support 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 initialize 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 linkNoop;
}
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;
}
Additional requirements for link support
Overview
The following are two important requirements:
- analog I/O
- link support can return raw value. Support must convert to engineering units.
- link support can return engineering units
- output records
- If possible link support should provide initial value during initialization
- If output can be modified other than by record itself than the records balue should be modified without causing a new output
Lets consider each of these separately.
Analog I/O
In V3 the aiRecord and aoRecords are responsible for conversion between raw values and engineering units. Raw values are 32 bit integers. Device support is responsible for setting the slope and intercept fields of the record. recordSupport takes care of the actual conversions. Device support can tell recordSupport that it set the engineering units rather than a raw value by a different return value.
For V4 the following is done:
aiRecord and aoRecord both have the following fields:
menu(menuConvert) {
choice(menuConvertRaw, "None")
choice(menuConvertLinear, "Linear")
choice(menuConvertTable, "Table")
}
struct(LinearConvert) {
field(engUnitsHigh,float64)
field(engUnitsLow,float64)
field(slope,float64)
field(intercept,float64)
}
...
field(convert, menu(menuConvert))
field(linearConvert, struct(LinearConvert))
field(convertTable,string);
field(units, string)
NOTE This section is written as though record support itself does the conversions. Most record support would call the standard support for LinearConvert.
If a record instance has convert set to None then recordSupport does the following.
- if link support implements LinkFloat64 it just calls this to obtain an engineering units value
- otherwise it gets a value via interface LinkInt32 and converts the value from in int32 to a float64.
If a record instance has convert linear
- if link support implements LinkInt32 it uses LinkInt32 otherwise it uses LinkFloat64
- it uses slope and interccept to convert from raw to engineering units
LinkInt32 is defined as:
interface LinkInt32 {
LinkResult get(Callback callback,Int32 data);
LinkResult put(Callback callback,Int32 data);
boolean getBounds(Int32 low, Int32 high);
}
If convert is linear and getBounds returns true than recordSupport uses the returned value to compute slope and intercept. In all other cases recordSupport just uses whatever values they have.
For example support for a 16 bit unipolar ADC would set low=0, high=65535, and return true. If link support can not determine the bounds then getBounds returns false.
NOTE:The interface implemented by breakpoint tables must be defined.
It will support the same semantics as the V3 breakpoint tables.
Output Records
The following are two features that link support should implement whenever possible:
- When a record is initalized provide the current value of the output
- If the actual output is changed by something other than the output record than the output record should also be modified to reflect the new output value.
The first feature is especially desirable when an IOC is rebooted. If a hardware output can survive a reboot this is a very desirable feature. If the output is to a field bus, a PLC, or to a record in another IOC it is often possible for the link support to determine the current value.
The second feature is something that was often asked for in V3 but only implemented for some special cases. This is especially desirable when the output is connected to a system that can have multiple sources of control. Examples are GPIB devices with front pannel controls, PLC systems that have manual controls or non-epics network access.
For V3 some of the hardware , but none of the soft, support implemented the first feature but by modifying actual fields in the record. V3 did not implement the second feature. Some special device support, not part of base, was implemented to support this feature.
For V4 these features are supported as follows:
record initialization
The link support implements an interface that has a put and a get method. For example:
interface LinkInt32 {
LinkResult get(Callback callback,Int32 data);
LinkResult put(Callback callback,Int32 data);
boolean getBounds(Int32 low, Int32 high);
}
If the link support can support readbacks then it implements get and returns linkNoop. If it does not support readbacks then it implements get by just returning linkNoop.
monitors for outputs
If output link support can detect when something besides the link support itself modifies the output then it implements interface MonitorLink
interface SupportMonitor {
void addMonitor(Callback callback);
void removeMonitor();
}
At initialization recordSupport looks for this interface and if found it calls addMonitor. The link support calls the callback whenever it detects a change in the output that it did not cause. recordSupport causes itself to be processed without making a new call to the link support. ---
