Data Acquisition

Figure 8.2GPIB connector (IEEE 488) and pin assignmentsThe 24 lines, in each cable, consist of 8 data lines and 8 pairs (16) of control and busmanagement lines. The data lines are used exclusively to carry data, in a parallel configuration,one byte at a time, along the bus. The control and bus management lines areused for various bus management tasks that synchronize the flow of data. When data orcommands are sent down the bus, the bus management lines distinguish between the two.The GPIB uses binary voltage signals to represent the information that is carried on thelines of the bus. It uses the symbols ‘true’ and ‘false’ to represent the two states ofvoltages on the lines. The GPIB uses the logic convention called ‘low-true’ or negativelogic, where the lower voltage state is ‘true’ and the higher voltage state is ‘false’.Standard TTL voltage levels are used. For example, when a line is true, or asserted, theTTL voltage level is low (≤ 0.8 V), and when the line is ‘false’, or unasserted, the TTLlevel is high (≥ 2.0V). The ‘low-true’ logic means that any device can set a bus controlvoltage ‘low-true’, but no line can be at a voltage ‘high=false’ unless all the devices onthat line allow it to go ‘high=false’.8.3 Physical connection configurationsThe devices on the GPIB can be connected in a star configuration, as shown in Figure 8.3,or in a chain (linear) configuration as shown in Figure 8.4.A star configuration is one where each instrument is connected, by means of a separateGPIB cable, directly to the controller. The connectors are all connected to the same portat the controller. A drawback to this simple configuration is that all of the devices on thebus must be relatively close to the controller because of the length limitation of eachcable.