5 The GP Interface Bus

Transcription

1 5 The GP Interface Bus In this chapter... Introduction to the System Interface Buses, 5-2 The GPIB Statements, 5-3 Addressing the Bus And Bus Devices, 5-5 A Reference Description of the GPIB, 5-8 Objectives When you finish reading this chapter, you should be able to: Understand the General Purpose Interface Bus (GPIB) Prerequisites There are no prerequisites associated with using this chapter. Required Tools and Materials There are no required tools or materials associated with using this chapter. Agilent Technologies 2001, 2003 BT-BASIC Programming /2003

2 Introduction to the System Interface Buses The test system can be interfaced to busable instruments and devices via the General Purpose Interface Bus (GPIB). The GPIB Bus The GPIB bus is used for communication between the workstation and: DUT power supplies in the testhead, Any optional DUT power supplies in the support bay, and. Table 5-1 Function Listener Talker Device functions on the GPIB Description A device capable of receiving data from other devices on the bus. A listener might be a printer, a programmable power supply, etc. A device capable of transmitting data to other devices on the bus. A talker might be a voltmeter or a counter. Any optional instrument accessories. Device Functions on the GPIB Devices on the GPIB (Table 5-1)communicate by way of three basic functions, and each busable device is equipped with the hardware and firmware necessary to carry out one or more of these functions Controller A device capable of managing communications over the GPIB. This control includes such things as addressing, and ending commands. In the test system, only the workstation is capable of this function. So for the remainder of this chapter, when we say controller we are referring to the workstation's function as an GPIB controller. Be aware that some devices on the bus may be able to function only as listeners, or only as talkers, while other devices may be equipped to function as both. Agilent Technologies 2001, 2003 BT-BASIC Programming 5-2

3 The GPIB Statements As a test system user, your interaction with GPIB is through BT-BASIC statements which either transfer data via the bus or directly manipulate operation of the bus. Both types of statements are described below. General Purpose I/O Statements Used With GPIB Some of the general purpose BT-BASIC I/O statements described in Chapter 4, Input / Output can be used to communicate with devices on the GPIB bus. These statements are described in Table 5-2. GPIB-specific tasks. If you need to know more about the data and control lines these statements manipulate, refer to A Reference Description of the GPIB on page 5-8. The dedicated GPIB statements are described in Table 5-3 on page 5-4. Table 5-2 I/O statements used with GPIB Statement assign to enter and enter using output and output using Description Assigns (at-name) to the bus or to a device. Enables sequential access. Inputs data from devices Outputs data to devices The Dedicated GPIB Statements BT-BASIC also contains several statements, referred to as dedicated GPIB statements, which perform Agilent Technologies 2001, 2003 BT-BASIC Programming 5-3

4 Table 5-3 Dedicated GPIB statements Statement abort clear local local lockout ppoll remote send spoll srq timeout trigger Description Clears the bus to its initial state by activating IFC for at least 100 microseconds. Also clears ATN and sets REN. If the bus is addressed, issues device clear (DCL). If a device is addressed, issues selected device clear (SDC). Sends go to local (GTL) to a selected device, or clears REN if the bus is addressed. Sends local lockout (LLO) to the bus and prevents any device from being operated locally. Conducts a parallel poll on the bus and returns the result as a function value. NOTE: This statement is not supported with the 82357A USB/GPIB adapter in the IPC, or the HP 9000/712 controller. This statement can only be used on an optional auxiliary bus. Sets the Remote ENable line (REN) to 1. If a device is addressed, the device is also addressed to listen. Sends commands and data directly to the bus. Conducts a serial poll for a specific device on the bus. Returns the current value (0 or 1) of the SRQ line. Sets timeout for a bus or device. Issues Group Executive Trigger (GET) which triggers devices to start their (device-dependent) activities. If the bus is addressed, all current listeners are triggered. If a device is addressed, just that device is triggered. Agilent Technologies 2001, 2003 BT-BASIC Programming 5-4

5 Addressing the Bus And Bus Devices Because the GPIB and the devices connected to it are part of the file system, they are addressed as if they were files. Either a <file id> or can be used to address the bus or a device on the bus. is generally preferred because it is faster and because the device remains open and can be accessed sequentially. When the GPIB is installed, it is associated with a file in the $AGILENT3070_ROOT /dev directory. The file name for a GPIB interface is $AGILENT3070_ROOT /dev/hpibx, where x is the number of the GPIB bus. Because the GPIB is bus #1, its associated file is $AGILENT3070_ROOT/dev/hpib1. How Specific Devices are Addressed The syntax of an GPIB bus address, shown below, includes two levels of bus addressing: the complete bus, and an individual device on the bus. Be aware that not all GPIB statements can address both levels. SYNTAX <GPIB> <GPIB device> <GPIB> and <GPIB device> can be: <file PARAMETERS <file id> is a <string expression> <name> is a <label> <GPIB> addresses an entire bus. The file name is $AGILENT3070_ROOT/dev/hpibx, where x is the number of the bus. <GPIB device> is the file name to address a specific device on a bus. When a device is addressed, the following sequence of commands is sent on the bus: Unlisten the GPIB. Address system controller to listen or talk. Address device to talk or listen. Agilent Technologies 2001, 2003 BT-BASIC Programming 5-5

6 Example 5-1 Example of addressing specific devices clear btgetenv$( AGILENT3070_ROOT ) & "/dev/hpib7"! Clears GPIB bus #1 msi btgetenv$( AGILENT3070_ROOT ) & "/dev" to "dvm1"! to the DVM on bus #1 Voltage Reserved GPIB Addresses and Device Names Addresses and device names on the system dedicated bus are reserved and cannot be assigned to any other instrument, even if there is no instrument at a particular address. If these addressing requirements are not adhered to, the test results can be unpredictable. The reserved addresses and device names are shown in Table 5-4. Table 5-4 Reserved GPIB addresses and device names Dedicated Device File name (/dev/...) GPIB Address Hex Decimal Miscellaneous GPIB #1 hpib1 1F 31 Oscilloscope scope 7 7 HP 3488A hp Voltmeter dmm A 10 Reference Voltmeter dmm_ref B 11 Synthesizer synth Module 0, ASRU Card channels 1-4 ps Module 1, ASRU Card channels 1-4 ps Agilent Technologies 2001, 2003 BT-BASIC Programming 5-6

7 Table 5-4 Reserved GPIB addresses and device names (continued) Dedicated Device File name (/dev/...) GPIB Address Hex Decimal Module 2, ASRU Card channels 1-4 ps Module 3, ASRU Card channels 1-4 ps Module 0, ASRU Card channel 5 or 5-6 ps4 1A 26 Module 0, ASRU Card channel 6 ps5 1B 27 Module 1, ASRU Card channels 5-6 ps6 1C 28 Module 1, ASRU Card channel 6 ps7 1D 29 Module 2, ASRU Card channels 5-6 ps8 1 1 Module 2, ASRU Card channel 6 ps9 2 2 Module 3, ASRU Card channels 5-6 ps Module 3, ASRU Card channel 6 ps NOTE If a power supply is split between modules, use the device file and GPIB address appropriate for the connections in the lower-numbered module. Agilent Technologies 2001, 2003 BT-BASIC Programming 5-7

8 A Reference Description of the GPIB Even though communication over the GPIB is normally handled through BT-BASIC statements, it may be helpful for you to understand how the individual bus lines function. The following paragraphs provide an overview of the GPIB. For more detailed information on bus communications, see the IEEE Publication on bus communication standard The GPIB uses a 24-conductor cable to interconnect several devices in parallel. Eight of the lines are used as grounds, and sixteen are responsible for bus communications and control. The sixteen lines that carry information are usually the only communication link between the interconnected instruments. Eight of these lines transmit data and the other eight are responsible for timing and for control of data transfers. Negative logic (low true) is used on all sixteen of the lines. The data and control lines are illustrated in Figure 5-1 on page 5-9. Agilent Technologies 2001, 2003 BT-BASIC Programming 5-8

9 Figure 5-1 GPIB bus structure Device A Data Bus Data Lines Device B Device C Data Byte Transfer Control Handshake Lines Device D General Interface Management Control Lines D1/ DAV NRFD NDAC IFC ATN SRQ REN EO1 Agilent Technologies 2001, 2003 BT-BASIC Programming 5-9

10 The Data Lines Data is transmitted on the eight data lines (DIO-1 through DIO-8) as a series of eight-bit bytes. Seven of the eight bits represent an ASCII character, with the eighth bit available as a parity bit. Each data byte transfer is accomplished with an interlocked handshake technique which is controlled via the DAV, NRFD, and NDAC control lines. The Handshake Control Lines Three handshake control lines are used to control asynchronous data byte exchanges so that a talker will not send data faster than each of the listeners can accept it. The three handshake control lines are described in Table 5-5. Table 5-5 Line Handshake control lines Description Using these three lines, a typical data exchange between instruments proceeds as follows. All devices currently designated as active listeners indicate (via the NRFD line) when they are ready to accept data. A device that is not ready to accept data forces the NRFD line low (true), while a device that is ready for data allows the line to float high. Because the devices on the bus are wired in parallel, the device(s) pulling the line low override its floating high. This line remains low until all active listeners are ready to accept data. When the talker senses that the NRFD line has floated high, it places a data byte on the data lines and pulls the DAV line low. This action indicates to the listeners that the data byte on the data lines is valid and can be read. Each listener, operating at its own speed, then reads the data and lets the NDAC line go high. Only when all listeners have let the NDAC line float high will the talker sense that the data has been accepted. The talker can then let the DAV line go high to start the entire sequence over again and send the next byte of data. The Bus Management Lines DAV NRFD Data Valid Not Ready For Data The five remaining lines are responsible for bus management. They are described in Table 5-6. NDAC Not Data Accepted Table 5-6 Bus management lines Line ATN IFC Description Attention Interface Clear Agilent Technologies 2001, 2003 BT-BASIC Programming 5-10

11 Table 5-6 Line REN EOI SRQ Bus management lines (continued) Description Remote Enable End or Identify Service Request The bus management lines take care of bus protocol by: Clearing the GPIB to allow servicing of bus interrupts Telling devices if the information on the bus is data or a command Telling devices when a data byte on the bus is the last one of a data message The handshake control lines function interactively to accomplish a data byte or a command byte exchange on the data bus. By comparison, the bus management lines act independently of each other that is, each bus management line's function is specialized. Table 5-7 contains a brief explanation of the function of each of the management lines. Table 5-7 Bus management line function Line ATN (Attention) IFC (Interface Clear) Function Since both addressing information and data messages are sent over the same set of data lines, the ATN line is used to distinguish one from the other. When the ATN line is false, the eight-bit byte on the data bus is interpreted as a character in the data message. When the ATN line is true (low), the byte is interpreted as control or addressing information. Only the system controller can control the IFC line. When the IFC line is pulled low, all bus activity is unconditionally terminated, and the system controller takes control of the bus. Any current talker and listeners become unaddressed. Normally, this message is used only to abort an unwanted operation, or to allow the controller to regain control of the bus when something has gone wrong. Agilent Technologies 2001, 2003 BT-BASIC Programming 5-11

12 Table 5-7 Bus management line function (continued) Line REN (Remote Enable) EOI (End Or Identify) SRQ (Service Request) Function The REN line is used to allow instruments on the bus to be remotely programmed by the controller. Many GPIB instruments, such as digital voltmeters, can have various functions and ranges selected either locally by manipulating their front panel controls, or remotely by programming messages sent over the bus from the system controller. In order to remotely program an instrument that has both local and remote programming capability the REN line must be set. This enables the instrument to accept data from the data bus if it has been addressed to do so. Under normal conditions, the data sent to instruments over the bus are in ASCII format. At the end of an ASCII message, the linefeed character (LF = decimal 10) indicates the end of the data message. If a device wishes to send data in eight-bit bytes which represent general binary patterns (not ASCII characters), then the EOI line is used to mark the end of the data message. When a listener detects that the EOI line is set, it recognizes that the data byte on the bus is the last one in the data message. The controller is always in charge of the order of events on the GPIB. If a particular instrument on the bus has information of which the controller should be aware, it can pull the SRQ line low to ask for the controller's attention. This action represents a request for attention from the controller and not a demand. The controller determines when and how it will handle an SRQ. The instrument originating the service request will continue to do so until the request has been satisfied. What satisfies the SRQ for a particular instrument a voltmeter, counter, etc. can be found in the operating documentation for that instrument. Agilent Technologies 2001, 2003 BT-BASIC Programming 5-12