Analogy General Presentation


Introduction

Analogy is a free software project included in Xenomai which tries to provides management facilities for data acquisition devices in real time.

By data acquisition, we mean:

  • reading / writing analog signals
  • reading / writing digital I/Os
  • counting pulses and frequencies
  • generating pulses

By real-time, we mean:

  • Generally speaking: ensuring the various I/Os can be acquired and triggered in bounded delays
  • “Softwarely” speaking: providing a fixed determinism at any software layers (kernel drivers, user space applications).

Analogy is a fork of the Comedi project. Comedi is a DAQ framework originally developed by David Schleef. His goal was “to put a generic interface on top of lots of different cards for measurement and control purposes”. Comedi is mainly used with vanilla Linux and with RTAI.

Analogy relies on the Real Time Driver Model, which is “an approach to unify the interfaces for developing device drivers and associated application under Linux”. So, thanks to RTDM, implemented in Xenomai in a skin, the Analogy framework benefits from real-time determinism in both user and kernel spaces. That means that a Xenomai real-time task based on any skin (native, POSIX, etc.) can use Analogy without leaving the primary mode (hard real time constraints)

Note: we have quoted many parts of the comedilib documentation in this document.


DAQ related knowledge

Device Hierarchy

Analogy organizes all hardware according to the following generic

hierarchy:

  • Channel: the lowest-level hardware component, that represents the properties of one single data channel; for example, an analog input, or a digital output. Each channel has several parameters, such as: the voltage range; the reference voltage; the channel polarity (unipolar, bipolar); a conversion factor between voltages and physical units; the binary values “0” and “1”; etc.
  • Sub-device: a set of functionally identical channels that are physically implemented on the same (chip on an) interface card. For example, a set of 16 identical analog outputs. Each sub-device has parameters for: the number of channel and the type of the channels.
  • Device: a set of sub-devices that are physically implemented on the same interface card; in other words, the interface card itself. For example, the National Instruments 6024E device has a sub-device with 16 analog input channels, another sub-device with two analog output channels, and a third sub-device with eight digital inputs/outputs. Each device has parameters for: the device identification tag from the manufacturer, the identification tag given by the operating system (in order to discriminate between multiple interface cards of the same type), the number of sub-devices, etc.

Some interface cards have extra components that don’t fit in the above-mentioned classification, such as an EEPROM to store configuration and board parameters, or calibration inputs. These special components are also classified as “sub-devices” in Analogy.

Acquisition terminology

This Section introduces the terminology that this document uses when talking about “acquisitions.” Figure 1 depicts a typical acquisition sequence:

  • The sequence has a start and an end. At both sides, the software and the hardware need some finite initialization or settling time.
  • The sequence consists of a number of identically repeated scans. This is where the actual data acquisitions are taking place: data is read from the card, or written to it. Each scan also has a begin, an end, and a finite setup time. Possibly, there is also a settling time (“scan delay”) at the end of a scan. So, the hardware puts a lower boundary (the scan interval) on the minimum time needed to complete a full scan.
  • Each scan contains one or more conversions on particular channels, i.e., the AD/DA converter is activated on each of the programmed channels, and produces a sample, again in a finite conversion time, starting from the moment in time called the sample time in Figure 1 (sometimes also called the “timestamp”), and caused by a triggering event, called convert. In addition, each hardware has limits on the minimum conversion interval it can achieve, i.e., the minimum time it needs between subsequent conversions. Some hardware must multiplex the conversions onto one single AD/DA hardware, such that the conversions are done serially in time (as shown on the Figure); other cards have the hardware to do two or more acquisitions in parallel. The begin of each conversion is “triggered” by some internally or externally generated pulse, e.g., a timer.

In general, not only the begin of a conversion is triggered, but also the begin of a scan and of a sequence. Analogy provides the API to configure what triggering source one wants to use in each case. The API also allows to specify the channel list, i.e., the sequence of channels that needs to be acquired during each scan.

TODO: import the figure

DAQ functions

The basic data acquisition functionalities that Analogy offers work on channels, or sets of channels.

Single acquisition

Analogy has function calls to synchronously perform one single data acquisition on a specified channel: a4l_synch_read(), a4l_synch_write(). “Synchronous” means that the calling process blocks until the data acquisition has finished.

Instruction

An a4l_do_insn() instruction performs (possibly multiple) data acquisitions on a specified channel, in a synchronous way. So, the function call blocks until the whole acquisition has finished. In addition, a4l_do_insnlist() executes a list of instructions (on different channels) in one single (blocking, synchronous) call, such that the overhead involved in configuring each individual acquisition is reduced.

Scan

A scan is an acquisition on a set of different channels, with a specified sequence and timing. Scans are not directly available as stand-alone function calls in the Analogy API. They are the internal building blocks of an Analogy command (see below).

Command

A command is sequence of scans, for which conditions have been specified that determine when the acquisition will start and stop. A a4l_command() function call generates aynchronous data acquisition: as soon as the command information has been filled in, the a4l_command() function call returns, the hardware of the card takes care of the sequencing and the timing of the data acquisition, and makes sure that the acquired data is delivered in a software buffer provided by the calling process. Asynchronous operation requires some form of “callback” functionality to prevent buffer overflow: after the calling process has launched the acquisition command, it goes off doing other things, but not after it has configured the “handler” that the interface card can use when it needs to put data in the calling process’s buffer. Interrupt routines or DMA are typical techniques to allow such asynchronous operation. Their handlers are configured at driver load time, and can typically not be altered from user space.

Buffer management is not the only asynchronous activity: a running acquisition must eventually be stopped too, or it must be started after the a4l_command() function call has prepared (but not started) the hardware for the acquisition. The command functionality is very configurable with respect to choosing which events will signal the starting or stopping of the programmed acquisition: external triggers, internal triggers, end of scan interrupts, timers, etc. The user of the driver can execute a Analogy instruction that sends a trigger signal to the device driver. What the driver does exactly with this trigger signal is determined in the specific driver. For example, it starts or stops the ongoing acquisition. The execution of the event associated with this trigger instruction is synchronous with the execution of the trigger instruction in the device driver, but it is asynchronous with respect to the instruction or command that initiated the current acquisition.

Typically, there is one synchronous triggering instruction for each subdevice. Note that software triggering is only relevant for commands, and not for instructions: instructions are executed synchronously in the sense that the instruction call blocks until the whole instruction has finished. The command call, on the other hand, activates an acquisition and returns before this acquisition has finished. So, the software trigger works asynchronously for the ongoing acquisition.


Acquisitions and real-time

Thanks to RTDM, Analogy provides its services to both real-time and non real-time processes. Here, we will try to define the benefits of using an RT task with Analogy. We hope you will find it useful to find out your real needs.

Let’s start with an obvious point, Analogy displays the same features range in RT and NRT mode. So, there is no drawback at using real time services.

According to the chosen acquisition mode (synchronous or asynchronous), Xenomai brings different advantages.

Synchronous acquistions

As told earlier, an acquisition instruction is implemented thanks to a syscall (ioctl) which will select the suitable instruction handler in the driver space. This handler is made available by the driver and it is in charge of performing the whole acquisiton (configuration + trigger
data transfer). The acquistion completes with the syscall return from kernel space.

In this case, if the acquisition task runs in primary mode (managed by Xenomai’s shceduler), the syscall management will not suffer of unexpected preemption from Linux. So, the whole execution of the instruction execution will be as determinist as Xenomai allows it. For instance, if you want the instruction to be performed every 200 micro-seconds with an accuracy of +/- 20 micro-seconds, you can safely rely on Xenomai services; with Linux, you have to consider that the accuracy criteria will be broken sometimes.

Asynchronous acquisitions

The acquistion is performed in two major steps: firstly configure and trigger the acquisition and secondly transfer the acquired data.

For the first steps (configure and trigger), Xenomai will provide a better determinism than Linux does; so, for instance, if you want the trigger operation to be performed every 200 micro- seconds with an accuracy of +/- 20 micro-seconds, you can safely rely on Xenomai services; with Linux, you have to consider that the accuracy criteria will be broken sometimes.

For the second step (transfer), the better determinism provided by Xenomai can ensure you that no buffer overflow / underflow will happen. Xenomai will handle the DAQ device’s interrupts in bounded delays, so the buffer evolution will be closely managed. Consequently, Xenomai can allow you to allocate smaller transfer buffers.