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The IEEE 488 Bus

Here we look at a rather old but interesting bus - a bus intended for use in laboratory instrumentation.

The IEEE 488 bus dates from 1967 when the Hewlett Packard Company began to look for a standard bus to link together items of control and test instrumentation. The IEEE standard was introduced in 1976 and revised in 1978. An updated version of the standard IEEE 488.2 includes changes to the software environment but no significant modifications to the underlying physical layer. The IEEE 488 bus is known by several names: the General Purpose Interface Bus GPIB, the Hewlett Packard Instrument Bus HPIB, IEC 625-1 bus, the ANSI MC1-1 bus, or, more simply, the IEEE bus.

Up to 15 devices can be connected to the IEEE bus to enable them to exchange data at no more than 1 Mbyte/s over a maximum transmission path of 20 m or 2 m per device (whichever value is less). The figure illustrates the relationship between the IEEE bus, the IEEE interface, and the devices that communicate with each other via the bus. As this diagram demonstrates, the IEEE standard covers only the bus and the interfaces but not the devices connected to the interfaces. This distinction is important because we shall soon discover that the IEEE bus implements different communication methods between devices and between interfaces.

Up to now we have talked about bus masters and bus slaves. The IEEE bus supports three types of device: the controller, the talker, and the listener. A talker or transmitter is a device that can put data on the bus; a listener or receiver is a device that can read data from the bus; and a controller is a device that manages the bus and determines which device may talk and which may listen.

At any instant only one talker can send messages over the IEEE bus, but several listeners may be receiving the messages from the talker. The ability to support a single talker and multiple listeners simultaneously demonstrates a fundamental difference between typical backplane buses and the IEEE bus. Backplane buses transfer data between a master and a single slave, whereas the IEEE bus is able to transfer data between a master (talker) and several slaves (listeners) in one operation, which is, effectively, a broadcast mode of operation. Some modern backplane buses like the Futurebus+ can also operate in a broadcast mode.

Only one controller may be active at any given time, although not every implementation of the IEEE 488 bus requires a controller in, as talkers and listeners can be set up manually. That is, devices can be programmed as permanent talkers or listeners during their manufacture, or by means of switches on their panels. An active controller can give up control of the bus by permitting another controller to take control. In general, the controller is part of the host computer on which the applications program is being run. Furthermore, this computer invariably has the functions of controller, talker, and listener.

The IEEE bus uses 16 information lines that are divided into three distinct groups—the data bus, the data bus control lines and the bus management lines as the figure demonstrates.

Eight data lines, DIO1 to DIO8, transfer data between a talker and one or more listeners, or between the controller and some or all of the devices connected to the bus. Information transmitted over the IEEE bus falls into one of three categories: multiline messages involving the transfer of eight bits of device-dependent (i.e., application level) data on the data bus (DIO1 to DIO8), multiline messages involving the transmission of control information from the controller on DIO1 to DIO8, and uniline messages involving a message on one of the bus management lines. The expression device-dependent data means that the data is in a format that has a meaning only to the device using the IEEE bus. The data lines carry two types of information: bus control information and information sent from one bus user to another.

We can categorize the IEEE bus’s three modes of data transmission as follows:

1. A byte of user data or device-dependent data is called a multiline message and is transmitted over the 8-bit data bus. The device-dependent message does not directly affect the operation of the IEEE bus itself or the IEEE bus interface, and its meaning depends only on the nature of the devices sending and receiving it.

2. A byte of IEEE bus interface control information can be transmitted over the data bus. Control information acts on the interfaces in the devices connected to the bus or affects the operation of the devices in some predetermined fashion defined in the IEEE 488 standard.

3. A single bit of information can be transmitted over one of the five special-purpose bus management lines. Certain bus management lines may be used concurrently with the operations on the data bus.

Information flow on DIO1 to DIO8 is managed by three lines control lines, NRFD, DAV, NDAC (i.e., Not Ready For Data, Data Available, and Not Data Accepted). Three control lines are required because all data exchanges between a talker and one or more listeners are fully interlocked, and, if a talker is sending information to several listeners, the data is transmitted at a rate determined by the slowest listener. In general, the operation of the three data bus control lines is controlled by the bus interfaces in the devices connected to the bus, and is entirely transparent to the user. We will describe how these lines function later.

The five bus management lines, IFC, ATN, SRQ, REN, EOI, perform special functions needed to enhance the operation of the bus. In a minimal implementation of the IEEE 488 bus, only one bus management line, ATN, is absolutely necessary. The functions of the bus management lines are summarized as follows.

• ATN (attention) The ATN line distinguishes between two types of message on the eight data lines. When ATN is true (i.e., electrically low), the information on DIO1 to DIO8 is interpreted as a control message from the controller. When ATN is false, (i.e., logical zero or electrically high) the message is a device-dependent message from a talker to one or more listeners. Note that only the controller can assert the ATN line (or the IFC or REN lines).
• IFC (interface clear) The controller uses the IFC line to place the bus in a known state. Asserting IFC resets the IEEE bus interfaces but not the devices connected to them. After an IFC message has been transmitted by a controller for at least 100 ms, any talker and all listeners are disabled, and the serial poll mode (if active) is aborted.
• SRQ (service request) The SRQ line performs the same role as an interrupt request and is used by a device to indicate to the controller that it wants attention. The controller must perform a serial poll to identify the device concerned, using a specified protocol.
• REN (remote enable) The REN line is used by the controller to select between two alternative sources of device control. When REN is true a device is controlled from the IEEE bus, and when false it is controlled locally. In general, local control implies that the device is operated manually from its front panel. The REN line allows a device to be attached to the IEEE bus, or to be removed from it. In the world of automated testing, the assertion of REN turns a manually-controlled instrument into one that is remotely controlled.
• EOI (end or identify) The EOI line serves two, mutually exclusive, purposes. Although the mnemonic for this line is EOI, it is frequently written END (end) or IDY (identify), depending on the operation being carried out. When asserted by a talker, END indicates the end of a sequence of device-dependent messages. That is, when a talker is transmitting a string of device-dependent messages on DIO1 to DIO8, the talker asserts EOI concurrently with the last message (i.e., byte) to indicate that it has no more information to transmit. When asserted by the controller in conjunction with the ATN line, the EOI line performs the identify (IDY) function, and causes a parallel poll in which up to eight devices (or groups of devices) may indicate simultaneously whether they require service. The parallel poll is described later.

The IEEE Data Bus

Data transfers on the IEEE data bus, DIO1 to DIO8, are interesting because they involve a three-line, fully interlocked handshaking procedure. A three-line handshake is unusual (indeed it is patented by Hewlett Packard). The signals used by the IEEE bus are all active-low, with an electrically high level representing a logical zero and an electrical low level representing a logical one. Active-low signal levels make it possible to take advantage of the wired-OR property of the open-collector bus driver (i.e., if any open-collector circuit pulls the line down to ground, the state of the line is a logical one). The definitions of the three signals controlling data movement on the IEEE bus are as follows.

• DAV (Data Valid) When true (i.e., electrically low), DAV indicates to a listener or listeners that data is available on the eight data lines.
• NRFD (Not Ready For Data) When true, NRFD indicates that one or more listeners are not ready to accept data.
• NDAC (Not Data Accepted) When true, NRFD indicates that one or more listeners have not accepted data.

The timing diagram of a data transfer between a talker and several listeners is given below. Suppose the bus is initially quiet with no transmitter activity and that three active receivers are busy and have asserted NRFD to inform the transmitter that they are busy. In this state, the NRFD line will be pulled down by open-collector bus drivers into a logical one state (remember that the IEEE bus uses negative logic in which the true state is the electrically low state).

Multiplexing address and data

When one of the listeners becomes free, it releases (i.e., negates) its NRFD output. The negation of NRFD by a listener has no effect on the state of the NRFD line, as other listeners are still holding it down. This situation is shown by dotted lines. When, at last, all listeners have released their NRFD outputs, the NRFD line rises (electrically) to a logical zero state (signifying that the listeners are all not Not Ready For Data—that is, they are ready for data). Now the talker can go ahead with a data transfer.

The talker places data on DIO1 to DIO8 and asserts DAV after a 2 ms data-settling time. As soon as the listeners detect a logical one (i.e., low level) on DAV, they assert NRFD to indicate that they are once more busy.

Meanwhile, the listeners clamp their NDAC outputs electrically low (i.e., NDAC asserted) to indicate that they have not accepted data. When a receiver detects that DAC has been asserted, it reads the data off DIO1 to DIO8 and negates its NDAC output. That is, if its Not Data Accepted output is false, then it must be signifying data accepted.

Because all listeners must make their NDAC outputs false before the NDAC line can rise to an electrical high state (i.e., logical zero state), the talker does not receive a composite data-accepted signal until the last listener has released NDAC. The talker terminates the data transfer cycle, when it releases DAV and the receivers release NDAC in turn.

Configuring the IEEE Bus

Before the IEEE bus can be used by the devices connected to it, the controller must first assign one device as a talker and one or more devices as listeners The controller communicates with all other devices either by uniline messages (asserting one of the bus management lines), or by multiline messages (asserting ATN and transmitting a message via DIO1 to DIO8). Multiline messages can be further subdivided into those intended for all devices (universal commands) and those intended for specific devices (addressed commands). Remember that all messages use only 7 bits of an 8-bit byte, enabling 7-bit ISO characters to be assigned to the control messages.

Three multiline messages are used by the controller to configure talkers and listeners on the bus: MLA (my listen address), MTA (my talk address) and MSA (my secondary address). Consider first the action of the MLA command. Before a device may listen to device-dependent traffic on the bus, it must be addressed to listen by the controller. The 31 my listen address codes from 00100000 to 00111110 select 31 unique listener addresses. Each listener has its own address, determined either at the time of its manufacture or by manually setting switches, generally located on its rear panel.

By sending a sequence of MLAs, a group of devices can be configured as active listeners. The thirty-second listener address, 00111111, has a special function called unlisten, UNL. Whenever the UNL command is transmitted by the controller, all active listeners are disabled. By convention, an unlisten command is issued before a string of MLAs and has the effect of disabling any listeners previously configured for some other purpose.

Having set up the listeners, the next step is to configure a talker, which is done by transmitting an MTA. There are 31 my talk address codes from 01000000 to 01011110. As only one device can be the active talker at any given time, the act of issuing a new MTA has the effect of automatically disabling the old (if any) talker. The special code 01011111 is called UNT (untalk) and deactivates the current talker. Once a talker and one or more listeners have been configured, data can be transmitted from the talker to the listener(s), at the rate of the slowest device taking part in the exchange, and without the aid (or intervention) of the controller. The format and interpretation of this data is outside the scope of the IEEE 488 standard, but, as we have said, is frequently represented by ISO (ASCII) characters. Note that the controller is acting as an intermediary between talkers and listeners, in contrast to other buses in which potential talkers and listeners are usually autonomous.

Serial and Parallel Polling

Like many other buses, the IEEE 488 bus provides facilities for devices to request service from controllers (i.e., an interrupt mechanism). The IEEE bus supports two forms of supervisor request—the serial poll and the parallel poll, although the parallel poll cannot strictly be classified as an interrupt.

A device connected to the IEEE bus can request attention by asserting the SRQ (service request) bus management line. The controller detects the service request and may respond by initiating a serial poll. A service request, in IEEE bus terminology, corresponds to an interrupt request in conventional computer terminology. As the controller does not know which device initiated the service request, it must poll all devices sequentially. The recommended sequence of actions that should be carried out by the controller in response to a service request is:

Serial-poll

   Unlisten all active listeners with UNL

    Enable serial poll with SPE

    REPEAT

        Transmit a MTA to a device to be polled

        Read the response from the polled device

    UNTIL all devices polled

    Disable serial poll with SPD

    Untalk all devices with UNT

End serial_poll

After the controller has entered the serial poll mode, it transmits successive talk addresses, MTAs, and examines the resulting requested service messages from each of the devices addressed to talk, until an affirmative response is obtained. The controller ends the polling sequence by an SPD (serial poll disable) command.

A parallel poll is initiated by the controller and involves several devices concurrently. The controller sets up the parallel poll by assigning individual data bus lines to devices (or groups of devices). For example, device 5 may be told to respond to a parallel poll by asserting DIO3. Then, the controller initiates the parallel poll and the configured devices respond.

The parallel poll is carried out by the controller asserting the ATN and IDY (identify) lines simultaneously. Whenever the IEEE bus is in this state with ATN and IDY asserted, the predetermined devices place their response outputs on the assigned data lines and the controller then reads the contents of the data bus. A parallel poll can be completed in only a few microseconds unlike the serial poll.

The IEEE 488 bus is an old bus that is still going strong because of its widespread use in the world of computerized instrumentation and test equipment. In many ways, serial buses such at the Ethernet, USB, FireWire, and even wireless LANs have left it behind. We have included it because it incorporates several interesting ideas such as the three-wire handshake.