Bus systems and communications

Transcription

1 Bus systems and communications CHRISTIAN ANTFOLK Announcement Assignment 1 Hand in on Monday Kursombud Data Lab 2 will be Onsdag den eller Torsdag den kl 8-12 or Lecture 9 on will be held in E:1145 1

2 Structure of a measurement system Physical quantity pressure temperature speed angular velocity luminosity force Measurement system Signal conditioning DAQ-cards Bus control of instruments GPIB (parallel) RS232 (serial) Bus systems with integrated and standardized instruments VXI/PXI Real time controllers Field buses Presentation (and control) Graphical programming LabVIEW Agilent VEE DASYlab Textual programming LabWindows CVI Measurement Studio Visual Basic Visual C/C++ Matlab What is a computer bus? A collection of wires which transfer digital data according to a specific protocol between separate units. There are several standards to allow seamless connectivity of instruments from a number of different vendors. E. g. PCI, USB, GPIB, Firewire, SATA, Ethernet, etc E. g. of entire systems with specific computer buses incorporated in each unit are fieldbuses, VXI-systems, PXIsystems, real-time controllers etc. 2

3 General Purpose Interface Bus Introduced by HP 1965 (HP-IB) 1 MB/s Requires special cables and och plug-in cards Max 20 m total cable length and 15 instruments Still very much used for instrument control in both industry and research environments, probably due to the rugged connectors IEEE-488 / IEEE General Purpose Interface Bus IEEE-488 GPIB-bus GPIB-instrument och controller Formalized the mechanical, electrical, and basic protocol parameters of GPIB. Software control Generalizerad instrument control IEEE SCPI VISA Basic syntax and format conventions, device-independent commands, data structures, error protocols. Standards for syntax and command for control of instruments. Makes it possible to programs independent of hardware interface (GPIB, serial, VXI ) 3

4 Connecting GPIB units Daisy chain Star GPIB system Three types of instruments (Controller, Talker, Listener) 31 primary addresses (Talker resp. Listener) 8 bit parallel data transfer Max. 2 m cable per instrument connected to the bus Negative true logic 4

5 GPIB system Handshake & Bus management lines Handshake DAV - Data Valid used by the Talker to indicate that the eight data lines are settled and valid NRFD - Not Ready For Data used by receiving unit (Listener) NDAC Not Data Accepted - used by receiving unit (Listener) Bus management ATN - Attention used by the controller IFC - Interface Clear - used by the controller, resets all connected units REN - Remote Enable - used by the controller, puts units in a remotely controlled state EOI - End Or Identify - used by the talker or the controllers. Talker : indicates the end of a message. Controller : start a parallell poll. SRQ - Service Request - used by the talkers to notify the controller that somethig is wrong 5

6 Two types of commands ATN is true (= low level = 0) bus commands can be sent E.g. put unit in Listener mode ATN is false (= high level = 1) data is sent to and from units through the DIO 1-8 lines The data can be instrument commands or measurement data Handshaking protocol

7 Handshaking protocol By transferring data in this way the data is placed onto the bus at a rate which is suitable for the talker, and it is held until the slowest listener has accepted it. In this way the optimum data transfer rate is always used, and there are no specifications and interface problems associated with the speeds at which data must be transferred. Bus commands 7

8 Example of instrument control over GPIB Mätdator : talaradress U / lyssnaradress 5 Spänningsaggr: lyssnaradress 3 Voltmeter: talaradress Q / lyssnaradress 1 Example of instrument control over GPIB continued Mätdator : talaradress U / lyssnaradress 5 Spänningsaggr: lyssnaradress 3 Voltmeter: talaradress Q / lyssnaradress 1 8

9 Example in LabVIEW Errors What is an error? An unknown command is sent Overflow (measurement out of range) What happens? The instrument activates the SRQ control line The instrument puts bits in the its status register What if you want the controller to handle the error? Poll 9

10 Polling Serial polling The controller addresses the units sequentially and they reply by sending back a status byte Parallel polling Each instrument is assigned one of the eight data lines. When the controller activated EOI (with ATN line set to true) the instrument(s) which generated a SRQ asserts its line. Bus commands IEEE Universal commands - unaddressed LLO - Local Lock Out Turns off the instruments front panel DCL - Device Clear Resets all units on the bus PPU - Parallell Poll Unconfigure Turns off the parallel poll function in units with that function SPE - Serial Poll Enable Activates serial poll function in units with that function SPD - Serial Poll Disable Turns off serial poll function in units with that function UNL - Unlisten - Recalls all Listeners on the bus UNT - Untalk Recalls all Talkers on the bus Addressed commands GET - Group Execute Trigger Eg. starts a measurement in an instrument SDC - Selected Device Clear Resets a specific device GTL - Go To Local Activates front panel on a specific device PPC - Parallell Poll Configure Prepares a listener for configuring of parallel poll TCT - Take Control Gives control to a specific device, device needs to have controller capability 10

11 Serial buses Standards for serial communication RS-232 Unbalanced (one ground wire + one active wire) Point-to-point Up to 19,2 kbit/s at 15 m cable RS-422 Balanced (both wires are active but in opposite phase) Point-to-point Up to 2 Mbit/s RS-485 Balanced (both wires are active but in opposite phase) Multiple units are connected in parallel, however the communication is serial (Multidrop) Up to 10 Mbit/s USB, FireWire, Ethernet, SPI, I2C 11

12 Comparison RS232 RS422 1 Tx 0 GND RS232 Tx+ RS422/RS485 Tx- 12

13 SPI Serial Peripheral Interface Typical serial communication between a AD converter and microcontroller Hardware addressing I 2 C Inter-Integrated Circuit 7 or 10 bit addressing Device need unique address Speeds up to 5 Mbit/s 13

14 Serial communication USB, FireWire, Ethernet 5 m cable for USB, 5 Gbit/s (USB 3), FireWire 72 m cable 3.2 Gbit/s Ethernet 72 m cable 10 Gbit/s SATA 3 8 m cable Up to 6 Gbit/s Designed to send data quickly to harddrives Drawbacks From an industrial perspective No (standard) secure locking mechanism on the connector Cables (usually) not specified for (heavy) industrial use 14

15 Connectors on a instrument Universal Serial Bus - USB Host controlled (computer controls how connected units communicates) A host can address up to 127 units 1,5, 12 and 480 Mbit/s Available on most (if not all) modern computers A B Lund University Faculty of Egineering Dept. of Biomedical Engineering 15

16 Example of a USB based system Example of a USB based system 16

17 USB Data transmission 1. The USB-unit is connected 2. Host/Controller gives the unit an address 3. A reset command is sent to the unit 4. The data transfer speed of the unit is checked Low speed = 1.5 Mbit/s Full speed = 12 Mbit/s High speed = 480 Mbit/s (USB 2) SuperSpeed = 5 Gbit/s (USB 3) SuperSpeed+ = 10 Gbit/s (USB3.1) 5. The different endpoints of the unit is checked, how much power it uses etc. 6. The internal clock of the unit is synchronized to the host 7. Token packets are sent before the command/data packets are sent 8. Data packets are always followed by handshake packets USB Data Transmission Token packet Data packet (optional) Handshake packet 17

18 USB Data transmission Packet IDs Enumeration : a host give a device an address 1-127, the device sens a device descriptor FireWire IEEE 1394 Daisy chain topology Support peer-to-peer (no controller needed) Up to 63 devices Bitrate : Mbit/s (FireWire S3200) Many computers do not have FireWire License fee Complexity of devices = less devices on the market 18

19 LAN & LXI (LAN extension for Instrumentation Rackinstruments VXI & PXI 19

20 VXI VMEbus extensions for Instrumentation Faster & more compact than GPIB (160 MB/s) Open standard (1987). Backed by more than 250 manufacturers Can use MXI (Multisystem extension Interface) or GPIB if traditional instrument are also used in the same setup FireWire (IEEE-1394), USB, LAN etc The PCI bus in a PC The PCI-bus, 32 bits, 133 MB/s, 33 MHz Peripheral Component Interconnect The PCI-e bus, PCI-express, 64 bits, 256 MB/s per line (total of 20 lines), 2 GHz Full duplex = to send and receive data at the same time => 512 MB/s 20

21 PXI PCI extensions for Instrumentation PXI PCI extensions for Instrumentation PCMCIA/PC card/pc express card to laptop MXI-buss to stationary PC Between two PXI system Fastest with an embedded controller 21

22 PXI Express Bandwidth > 6 Gbyte/s Vectorsignalgenerators (up to 6,6 GHz) Vectorsignalanalyzer Fieldbuses Used to interconnect automation devices in a network Heavily used in industry 22

23 Fieldbuses Fieldbuses 23

24 Examples of Fieldbuses FIP (Factory Information Protocol) Three transmission speeds : kbit/s, 1 Mbit/s, 2.5 Mbit/s Max 255 units per segment The bus arbitrator sends an address and the corresponding device answers by outputting a value Profibus RS-485 or optical Max 122 units 500 kbit/s Token bus : the active units pass around a token, passive units only listen CAN-bus (Controller Area Network) Developed by Bosch for vehicles Max 1 Mbit/s at 40 meter Bus organization: no superior node, messages lack address each node is programmed to receive a message of a specific type The CAN bus in cars The expenses for the electronic circuitry in today s cars can be up to a fourth of the construction costs Advanced cars: up to 4 km cable (Apollo : 150 kb memory ) Weight and power consumption effect the gasoline/diesel consumption Cars of today have a number of different buses of different data transmission rates < 125 kbit/s - comfort (seats, windows, AC etc) < 1 Mbit/s more real time demanding functions like ABS and cruise control 24

25 Why use fieldbuses Distributed intelligence gives: Less cabling, especially over long distances Measurement cells can be made self calibrating or be calibrated remotely through the bus Self diagnostic systems Flexible system when transducer units are exchanged Buses Local Regional National 25

26 Local bus SPI I 2 C Regional bus USB GPIB PXI etc 26

27 National bus Ethernet Fieldbus Comparisons 27

28 Comparison Projects 2014 Project: Build a measurement system (room E:1309B) Choose project week 4 of the course Short project description and suggested approach to solution Monday Short oral presentation Friday written report Report hand-in Friday students per group Gear : PC with LabVIEW and Matlab 28

29 Example projects FPGA/CompactRIO-based measurements (industry) Automated Voltage vs. Frequency measurement for an acoustophoresis setup (BME) Control of pneumatic actuators for stimulation in an fmri environment. (Radiation Physics / BME) 29