Two types of information can be transferred over the GPIB bus: commands and data. When both the controller drives Attention (ATN) and Data Valid (DAV) low, the byte value on DIOL-D108 represents a command to one or more devices. Such commands enable remote operation of devices on the GPIB and assign them to be talkers or listeners, and so on.

When ATN is high and DAV is low, the byte value on the data bus is data. Hence, ATN is a switch that identifies whether the data bus value is a command or data; DAV low means what its name says-the data on DIO1-DIO8 is valid for all listeners to read.

All bus transfers-both controller commands and talker-listener data transfers- take place using Hewlett-Packard's patented three-wire handshake. All devices must handshake when commands (ATN low) are being sent, but only listeners handshake for data transmission. This allows high-speed transmission between two fast devices even when there are much slower devices present on the same bus. The figure below presents a timing diagram for a command followed by a 1-byte data transfer, showing how the handshake signals work.

To better understand how the handshake operates, here is the sequence of events that occurs when the controller sends a command to the devices on the bus:

Before putting a new command on the bus, the controller checks to see if the Not Ready For Data (NRFD) line is high. Any device that's not ready to accept another data byte holds NRFD low. Thus, thanks to the open-collector connection of the NRFD line, it can go high only when all devices are ready to accept a new command byte.

The controller sets ATN low to indicate a command is being sent, places the command code on the data lines, and, after a delay to allow the DIO lines to settle, pulls DAV low to indicate that a valid command is present on DIO1-D108.

When each device sees DAV go low, the device pulls its NRFD line low to indicate that it knows a new byte is present, but that it hasn't yet received and stored it.

Once each device has stored the command byte, it releases the No Data Accepted (NDAC) line to indicate that it's accepted the byte. When the slowest device has released NDAC, this open-collector line will finally go high.

The controller now knows that all devices have accepted the command, so it sets DAV high and removes the command byte from the data lines.

On seeing DAV go high, each device sets its NDAC line low again so that it's in the proper state for the next data transfer.

When it's handled the command just received and is ready to receive the next byte, each device releases its NRFD line. As a result, NRFD will finally go high when the slowest device is ready.

The same sequence of events occurs when a talker sends data to one or more listeners. The only difference is that for a data transfer the ATN line is high, and only those devices that are currently configured to be listeners participate in the handshake. Non-listeners do not drive the NRFD and NDAC lines.

You may be wondering why such a complex handshake is used here. The answer is that in nearly all other bus systems such as ISA or SCSI, data is sent from a single source to a single receiver. In a GPIB system, however, there can be more than one listener and the three-wire system prevents multiple acceptance of data by a fast listener while a slow one is still busy accepting the data.

When messages (either commands or data) are sent from one GPIB device to another, the programmer doesn't need to worry about the details of the handshake. GPIB interfaces in instruments and GPIB controller cards for PCs use sophisticated ASICs (application specific integrated circuits) that constitute a complete (or nearly so) GPIB interface on a chip.

802.11x, also sometimes known as Wi-Fi, is an IEEE certified wireless networking standard that currently includes the IEEE 802.11a, 802.11b and 802.11g specifications. In the U.S., the RF emission of these devices is governed by FCC Part 15 rules. These rules govern the power output, equipment and antenna configurations useable in the unlicensed bands.

802.11x is an extension of wired Ethernet, bringing Ethernet-like principles to wireless communication. As such, 802.11 is agnostic about the kinds of data that pass over it. It’s primarily used for TCP/IP, but can also handle other forms of networking traffic, such as AppleTalk or NetBEUI