Overview of IEEE-488 bus
IEEE-488 bus is a parallel bus interface standard. IEEE-488 bus is used to connect systems, such as microcomputers, digital voltmeters, digital displays and other instruments and meters can be assembled with IEEE-488 bus. It transmits signals in bit parallel and byte serial bidirectional asynchronous mode, and the connection mode is bus mode. Instruments and equipment are directly connected in parallel on the bus without intermediate unit, but up to 15 devices can be connected on the bus. The maximum transmission distance is 20m, the signal transmission speed is generally 500kb / s, and the maximum transmission speed is 1MB / s.
IEEE488 is a kind of parallel external bus, which was developed by HP company in 1970s. In order to solve the connection trouble caused by the incompatibility between various instruments and computers, HP-IB bus is developed. In 1975, IEEE Recommended IEEE488 Standard bus. In 1977, International Electrotechnical Commission (IEC) also approved and recommended this bus, named iec-ib. So this kind of bus uses IEEE-488, iec-ib, HP-IB or GP-IB at the same time. Due to the introduction of IEEE488 bus, when using IEEE488 Standard to build a computer-controlled test system, there is no need to add a lot of complex control circuits. IEEE488 system uses rack stacked intelligent instruments as the main components to form an open building block test system. Therefore, IEEE488 bus is one of the most widely used communication buses in industry.
Working mode of IEEE-488 bus
The structure of IEEE488 bus interface is shown in Figure 8.22. Using IEEE488 bus, the microcomputer and other devices are connected together. Serial connection or star connection can be used.
Figure 8.22 IEEE488 bus interface structure
Each device in IEEE488 system can work in the following three ways.
(1) “Listener” mode this is a receiver that receives data from the data bus. A system can have more than two “listeners” working at the same time. The devices that can act as “listener” function are: microcomputer, printer, plotter, etc.
(2) Speaker mode this is a transmitter, which sends data to the data bus. A system can have more than two speakers, but only one speaker can work at any time. Equipment with “speaker” function include: microcomputer, tape drive, digital voltmeter, spectrum analyzer, etc.
(3) The “controller” mode is a device that issues commands to other devices, such as addressing other devices or allowing the “speaker” to use the bus. The controller is usually a microcomputer. A system can have more than one controller, but only one controller can work at any time.
Various devices on IEEE488 bus can have different functions. Some devices, such as microcomputers, can have the functions of controller, listener and speaker at the same time. Some devices only have receiving and sending functions, while some devices only have receiving functions, such as printers. At a certain time, the system can only have one controller, and when data transmission, at a certain time, only one transmitter can send data, allowing multiple receivers to receive data. That is, one to many data transmission can be carried out.
In general applications, for example, the data measurement system controlled by a microcomputer connects the microcomputer with various test instruments through IEEE488. At this time, only the microcomputer has three functions of control, sending and receiving, while other devices on the bus have no control function, but they still have receiving and sending functions. When the bus is working, the controller issues a command to specify which device is the transmitter and which device is the receiver. Then the transmitter can use the bus to send data, and the receiver receives data from the bus.
IEEE-488 bus usage protocol
(1) Data transmission rate ≤ 1MB / s.
(2) There are no more than 15 devices connected to the bus (including the microcomputer as the main controller).
(3) The maximum distance between equipments is less than or equal to 20m.
(4) The total cable length of the whole system is less than or equal to 220m. If the cable length exceeds 220m, the timing relationship will be changed due to time delay, resulting in unreliable operation. In this case, a modem should be attached
(5) All data exchange must be digital.
(6) The bus uses 24 wire combined plug socket and adopts negative logic, that is, the level less than + 0.8V represents logic “1”; The logic “0” is represented by a level greater than 2V.
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