Experiment 419: IEEE Standard Digital Interface for Programmable Instrumentation

Transcription

1 Experiment 419: IEEE Standard Digital Interface for Programmable Instrumentation Introduction The IEEE-488 interface or General Purpose Interface Bus (GPIB) is a standard system for the interconnection of programmable instruments and a controlling computer. It allows up to 15 devices to be connected to one another. A device may have one or more of the properties of controller (e.g. a computer), talker (sends out data) or listener (receives commands). A typical system is shown in Fig. 1. Controller (computer) Talker (e.g. counter) BUS Listener (e.g. function generator) Talker/Listener (e.g. DVM) Fig. 1 The bus consists of 24 lines, comprising 8 data lines, 3 data transfer control lines, 5 interface management lines and 8 ground lines. Data and commands are transferred by a 3-line handshaking system under control of the management lines, in a byte serial/bit parallel manner. The five management lines are as follows: ATN (Attention): Indicates whether the byte being placed on the bus is data or a command (or address). EOI (End or Identify): Indicates end of transmission. SRQ (Service Request): This line is set true by any device requiring service (e.g. after taking a reading). The controller uses this information to conduct a SERIAL POLL, addressing each device in turn to find which one requires service. IFC (Interface Clear): Sets all devices to a known initial state. 1

2 REN (Remote Enable): Allows a device to be remotely controlled (as opposed to LOCAL control via its front panel switches). The three data control (handshaking) lines are: NRFD (Not Ready for Data): This is held true until all listeners are ready for data (or commands). NDAC ( Not Data Accepted): This is held true until all listeners have accepted the data. DAV (Data Valid). Set true by a talker to indicate that data or a control byte is on the bus and available to listeners. The signals, cables, plugs/sockets etc. are all standardised so that any instruments (with interfaces fitted) may be directly connected to one another and to a computer (with a suitable interface). The standard functions that an instrument may have are as follows: (1) Source Handshake (SH): Permits a talker to transfer data to one or more listeners. (2) Acceptor Handshake (AH): Permits a listener to accept data from a talker. (3) Talker (and Extended Talker) T(ET): A device may be addressed and enabled to be a talker. (4) Listener (and Extended Listener) L(EL): A device may be addressed and enabled to be a listener. (5) Service Request (SR): The controller may be notified that action is required. (6) Remote/Local (RL): A device can be controlled remotely by the controller, or locally (by its front pane1). (7) Parallel Poll (PP): Up to 8 devices may be polled simultaneously to determine whether service is required. (8) Device Clear (DC): Clears device interface. (9) Device Trigger (DT): Initiates one or m ore devices into operation. (10) Controller (C): System controller. The capabilities of a given instrument are marked on its rear panel. For example the DVM in this experiment is marked SH1, AH1, T5, TE0, L3, LE0, E1, SR1, RL1, DC1, C0, DT1, PP. The numerals indicate different levels of capability. This DVM has most functions except that it cannot be used as a controller. (TE and LE refer to additional levels of talking and listening, e.g. where an additional register may be accessed). 2

3 Each instrument has device address between 0 and 30, which may be set by 5 switches on the rear panel. Device address 31 is not allowed, being used for a special UNTALK command. The controller has a default address of 0. The pc lab card in the computer is a full implementation of the IEEE standard and includes the following commands. It is operated from BASIC (having a ROM which translates the commands to interface functions). Initialise: Output: Input: Clear: Initialises card after switching on. CALL INIT (10PORT,MYADDR,SETTING) Sends data/commands to devices CALL OUT (ADDR,D$) Receives data from devices CALL INN (ADDR,D$) Clears interface CALL CLR Device clear: Clears specified device CALL DEVCLR (ADDR) Local Lockout: Disables front panel controls CALL LLO Local: Returns specified device to local control CALL LOCAL (ADDR) Parallel Poll: Returns 8-bit byte showing any service requests CALL PPOLL (response) Remote: Serial Poll: Trigger: Puts specified device into remote mode CALL REMOTE (ADDR) Returns 8-bit byte from specified device CALL SPOLL (ADDR,response) Triggers a reading from specified device CALL TRIGGER (ADDR) 3

4 Description of Instruments The experiment permits control of a Phillips function generator (type 5190) and a Solartron DVM (type 7150). Solartron DVM type 7150 (SHI, AH1, T5, TEO, L3, LEO, SRI, RL1, DC1, DT1, PP, C0) This instrument has a very full implementation of the IEEE standard. It measures voltage (ac and dc), current (ac and dc) and resistance over a wide range of values and also has an autorange facility. The address has been set to device number 3 Command format (see handbook for full details): C : Normal (0) or Calibration (1) G: sample (must be in sample mode first) M: Mode 0 V dc 3 I dc 1 V ac 4 I ac 2 kω J: n sets Parallel Poll to bit n. N: Display format 0 Numeric with literals (ea Vdc) 1 Numeric only (e.g ) R: Range V I R 0 autorange autorange autorange V (dc only) V V - 20 kω V kω V 2A 2 MΩ MΩ T: Sample mode (0) or Track mode (1) Q: SRQ on error (0) or SRQ on error/output available (1) e.g. M0N0R2T1Q1 sets V dc, literals, 2 V range, track mode and normal SRQ. Serial poll byte: SRQ on 0 Output available Remote / local 0 0 Error 4

5 Philips LF Synthesiser (AH1, L2, RL1) The front panel controls enable waveforms to be produced at frequencies in the range 1 mhz to 2 MHz (for sinewaves and squarewaves) and up to 100 khz for trianglewaves; ac voltages are selectable from 0 to 19.9 V and dc offsets from 0 to V. The device is basically a listener (L2) and cannot transmit data. All functions are programmable from the controller, and the front panel controls can be locked out (RL1). The address has been set to device number 4. Command format: Frequency: FXX.XXXXEOS At least one digit must be set, the values being always in khz. The decimal point may be omitted. ac/dc voltage: AXX.XD-XXEOS ac dc 3 ac digits must be set, with leading zeros set, and the first ac digit must be 0 or 1. The decimal point must be set (and not at the end). The 6th byte must be D. For +ve offsets the - digit is simply omitted. The decimal point is in the same position as for ac. Both digits should be set. Waveform type: Wn 1 sine 2 square 3 triangle 4 sinewave with external AM 5 triangle with external AM egs. F0l.1 sets frequency to 1.1 khz A11.5 D-11 sets a.c. amplitude 11.5 V, dc offset V. W 2 sets waveform to square. F5151A01.9D05W1 sets frequency 5151 khz, ac amplitude 1.9 V, sinewave. (The EOS character is CHRS(3), but is added automatically in the program). Description of the Program The disc will boot from drive A and loads GW BASIC and runs the program (IEEE-E). You have to specify if you are using CGA (on a 1512) or EGA (on a 1640). The device addresses are set to 3(DVM) and 4(synthesiser). The OPTION MENU is then displayed. OPTION MENU 1. Properties of IEEE interface 2. Frequency sweep 3. Amplitude sweep 4. End 5

6 Select 1 for the IEEE Interface Menu IEEE INTERFACE MENU 1. Print to device 2. Listen to device 3. Remote/Local 4. Trigger 5. SRQ and Serial Poll 6. Parallel Poll 7. Set time out 8. Initialise/Clear 9. Return to main menu 1) Print to Device. The device number is requested, and you are asked for a command. A command is entered as above, ea. SIT5DI5D0 or F2.2A0l.lD00. 2) Listen to Device. The device address is requested, and the reading returned immediately as a string, ea E1 DCV. 3) Remote/Local. This asks whether you want remote or local operation, and for the device address. 4) Trigger. The device address is requested and the device triggered to take a reading (this sets SRQ on). 5) Serial Poll. Only the DVM can initiate a service request. The SRQ line is examined and T or F returned. The poll on the DVM is then done, returning a one-byte response (see DVM section). 6) Parallel Poll. This returns a byte of value depending on the configuration set for the DVM (see DVM section). 7) Set Timeout. The timeout (in ms) is entered. 8) Initialise/Clear. The interface can be initialised, or all devices cleared. 9) Return to Main Menu. Procedure Examine the front panel controls on the DVM and Synthesiser and ensure that you can operate both instruments locally (set up a few waveforms and read the amplitudes on the DVM). Set the oscilloscope to dc, 1 V/cm, 2 ms/cm. Part I: Basic Properties of the GPIB (i) Program a square wave of amplitude 1.11 V ac, frequency 9.51 khz, dc offset -0.ll V. Write down the string required. Note that the front panel controls are disabled. 6

7 (ii) (iii) (iv) (v) Program a reading of this waveform on the DVM. Write down the strings used (for ac and dc) and the values obtained. Set Timeout to 2s and operate it by trying to listen to a device which is not connected (e.g address 5). Set DVM to SRQ on error. Generate an error by sending C1. Carry out a serial poll and record the result. Carry out a second serial poll and record the result. Check that these agree with the values expected from the serial poll byte. (Only the DVM has serial poll capability). Set DVM to Parallel Poll bit 1 by sending J1. Generate an error and carry out a Parallel Poll. Record the result. Carry out a second poll and record the result. Then carry out a serial poll followed by another parallel poll and record the final result. Part II: Automatic Measurement of Network Response. Connect the input of the network provided to the synthesiser, and the output to the DVM. Return to the Main Menu and select Frequency Sweep or Amplitude Sweep. The responses are stored in arrays and can be displayed on the screen or printed out. Run the programs to get an idea of the value of where the interesting ranges of frequency and amplitude lie. Then carry out specific runs to sketch plots of the following: (i) (ii) (iii) The frequency response well below amplitude limiting. The frequency response well above the start of amplitude limiting. The response at the resonant frequency as a function of amplitude. Note the peak responses and Q-factors in (i) and (ii), and the actual value where amplitude limiting starts in (iii). Sketch a possible circuit for the network. Note: Remember that the synthesiser accepts frequencies in khz. Discussion and Conclusions You should have discussed any necessary results in the appropriate sections above. Answer the following questions in addition to any general conclusions. (i) How many instruments can the bus handle (excluding the controller)? (ii) What is the form of the data/command transfer (e.g. serial bit, parallel byte etc)? (iii) How does the system tell whether a byte is data or command? (iv) Why is a three-line handshake required? (v) What are the relative advantages of (a) Serial Poll, (b) Parallel poll? (vi) What is meant by Local Lockout? (vii) What are the principal advantages of a standardised system such as the GPIB? (viii) What are the main disadvantages? 7