Frontend Operation: Difference between revisions

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==== Links ====
The midas documentation has moved. See [https://midas.triumf.ca/docs/develop/features/acquiring_data/frontends/ Frontends: overview] and [https://midas.triumf.ca/docs/develop/features/acquiring_data/frontends/programming/ Frontends: programming] for the new documentation about frontends, equipment and events.
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* [[Frontend Application]]
*
*
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== Introduction ==
 
The terms that are needed for a discussion of a <span style="color: green; font-style:italic; ">''frontend''</span> will be introduced here. The features of a typical <span style="color: green; font-style:italic; ">''frontend task''</span> will then be explained by reference to some of the templates included with the MIDAS package.
 
== Frontend  ==
 
The term "<span style="color: green; font-style:italic; ">''frontend''</span>" usually refers to a "<span style="color: green; font-style:italic; ">''frontend task''</span>" or program running on a particular computer which has access to hardware equipment in use by the experiment. An experiment may
run several <span style="color: green; font-style:italic; ">''frontends''</span>, each performing different functions.
 
A <span style="color: green; font-style:italic; ">''frontend application''</span> consists of 
* a fixed  experiment-independent system framework  (i.e. '''''mfe.c''''')
: handling the data flow control, data transmission and run control operation.
 
* a '''user part''' (e.g. '''''frontend.c''''')
: written by the user describing the sequence of actions to acquire the hardware data
 
A set of templates for the user part is provided in the MIDAS package (e.g. ''../midas/examples/experiment/frontend.c'').
 
The hardware access is only apparent in the user part ( referred to here as ''frontend.c'', but the user may select any name). The system framework is contained in the file ''mfe.c'', which is part of the MIDAS package (the name "mfe" stands for "MIDAS front end").
 
The term "<span style="color: green; font-style:italic; ">frontend code</span>"  in this document usually refers to the user part (i.e. ''frontend.c''), since the system framework (''mfe.c'') is fixed.
 
To build a <span style="color: green; font-style:italic; ">frontend task</span>, the user code ''frontend.c'' and system code ''mfe.c'' are compiled and linked together with the required libraries, by running a ''Makefile'' (e.g.  ''../midas/examples/experiment/Makefile'' in the MIDAS package).
A "<span style="color: green; font-style:italic; ">''frontend task''</span>"
may have any name the user chooses (e.g. ''fevme, fegpib''). To run the task, see [[Frontend Application]].
 
== Hardware Access ==
 
Examples of hardware modules that may be accessed by the [[#Frontend |Frontend]] are ADC modules, TDC modules, Scaler modules. See [[Supported Hardware]]; The frontend is usually linked to a [[device driver]] for the particular hardware in use. Device drivers (i.e. header and library file) for many common hardware modules ( VME, GPIB, RS232 and CAMAC) are provided in the MIDAS package (see [https://bitbucket.org/tmidas/midas/src/93fa5ed4a40df94d4241e4a1910e44d1c5c8b866/drivers/?at=develop Git Repository /Drivers]).
 
== Equipments and Events ==
In MIDAS,  "''hardware equipment''" refers to a collection of hardware (or devices) that are grouped together for convenience of readout, such as
* a set of high voltage supplies
* one or more crates of digitizing electronics e.g. ADCs and TDCs
* a set of scalers
 
Under MIDAS, ''hardware equipment(s)'' from the experiment are structured into software entities also called "''Equipments''".  From a software point of view, we keep this same term, i.e. "''Equipment''", to refer to the means of collecting the data related to hardware equipment. Thus the term "'''''Equipment'''''" refers to  ''a
single or a collection of sub-tasks meant to collect and regroup logical or physical data under a single and uniquely identified event''.
 
An "'''''event'''''" is thus a collection of data sent into the data stream by an ''Equipment''.
 
== Equipments in Frontends ==
 
A single [[#Frontend|frontend]] may contain several [[#Equipments and Events|equipments]]. For example, an experiment may have a frontend to service
crates of ADC, TDC and Scaler modules. The ADC and TDC modules may be grouped together in
one equipment, and the scalers in a second equipment.
 
In complex experimental setups, several frontends controlling different sets of hardware equipment may be attached
simultaneously to a given experiment, i.e. they share the same Online Database (ODB). For example, one frontend might
service some VME hardware (TDCs,ADCs,Scalers), while another might service some slow-controls hardware via GPIB.
 
 
== Frontend Features ==
 
A typical frontend will:
 
*    Register the given equipment list(s) to a specific MIDAS experiment.
*    Provide the mean of collecting data from the hardware source defined by each Equipment read function.
*    Gather these data in one of the supported formats (i.e. FIXED format or in MIDAS data bank(s) ) for each Equipment.
*    Send these data banks to the buffer manager either locally or remotely.
*    Periodically collect statistics of the acquisition task, and send them to the Online Database.
 
 
=== Data transfer from the frontend ===
The system frontend [[#Frontend |framework]] (''mfe.c'') sends events to the buffer manager and optionally a copy to the Online Database (ODB). A "Data cache" in the frontend and on the server side reduces the amount of network operations pushing the transfer speed closer to the physical  limit of the network configuration.
 
A frontend's data transfer can be '''optimized''' for all kinds of experiments. Those where  multiple small events are produced can have the data built into  "[[Super Event]]s", by removing the bank headers to reduce the amount of data transferred.
 
An  [[Event_Builder |"event builder"]] mechanism is also available,
that can combine events from many different frontends into a composite event.
 
=== Frontend event triggers ===
 
The  data collection in the frontend framework can be triggered by several mechanisms.
Currently the frontend supports five different kind of event trigger (see [[Equipment Flags]]).
 
; Periodic events :
Scheduled event based on a fixed time interval. They can be used to read information such as scaler values, temperatures etc.
; Polled events :
Hardware signal read continuously for a certain time period. If the signal is TRUE, the Equipment readout will be triggered.
; Interrupt events :
Generated by particular hardware device supporting interrupt mode  (e.g. LAM if CAMAC).
; Slow Control events :
Special class of events that are used in the slow control system.
; Manual events :
Triggered by a remote procedure call (RPC). The web interface provides an extra
button for manual triggering by the user.
 
Each of these types of trigger can be enabled or activated for a particular experiment's  [[Run States and Transitions |run or transition state]] or a combination of any of them.
 
Examples such as
* read scaler event only when running : or
* read periodic event if the run state is  not paused  and on all transitions
are possible.
 
A frontend can also cause a transition to be  '''delayed ''' until a particular condition is met (see  [[Run States and Transitions#Deferred Transitions |Deferred Transitions]]).
 
===Event Readout Functions===
Associated with each Equipment is an readout function which runs when an Equipment is triggered. This function performs the actions required at the Equipment trigger, for example reading out the Equipment data and packing it into banks.

Latest revision as of 22:00, 30 October 2026

The midas documentation has moved. See Frontends: overview and Frontends: programming for the new documentation about frontends, equipment and events.

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