What Is a VME Board?
VME (Versa Module Europe) boards were developed as boards that use the VME bus, a bus for CPUs.
The VME bus used in VME boards was originally developed for Motorola’s 68000 series CPUs, and was later adopted as a global technical standard by the IEC (International Electrotechnical Commission) and It was later standardized as a technical standard by the IEC (International Electrotechnical Commission) and IEEE (Institute of Electrical and Electronics Engineers: a standardization organization in the electrical and information fields).
It is currently used as a standard bus for 64-bit CPUs in addition to 16-bit and 32-bit CPUs.
Usage of VME Boards
A VME board has 9 or 20 VME connectors on the motherboard, which is the foundation for connecting multiple VME boards.
This motherboard is called a VME backplane board or VME backplane chassis, and its material and size are determined to maintain its performance and quality.
The VME backplane board is very important because it electrically and structurally connects multiple VME boards through which data is transmitted and received.
CPU boards and IO boards are inserted into slots on the VME backplane board.
Principle of VME Boards
The data transfer used on VME boards is based on the asynchronous inter-clocking method, which means that data is transmitted in synchronization with the system clock. This is different from the method of sending and receiving data synchronized with the system clock.
The master side repeats data transmission and does not complete the data transfer cycle until the slave side responds with an acknowledgement signal indicating that data reception is complete.
This is advantageous and easy to design for peripheral devices that do not have sufficient transfer speed, since data can be sent and received according to the timing of the slave device.
However, it is necessary to avoid infinite transfer cycles from the master. For this purpose, the VME bus has a timeout function.
The VME bus is also designed to handle data collisions between devices on the VME bus. An arbitration module is defined as a dedicated functional module for detecting and responding to collisions, and it must be present in every VME bus system.
Two methods are defined for detecting and responding to collisions. These are the round-robin method and the priority method.
Round Robin Method
Round-robin is a method in which each device on the VME bus is sequentially assigned a number based on the number assigned to the device. For example, if there are devices A, B, and C, and the first round is A, B, and C, the second round is B, C, and A, and the third round is C, A, and B. This method determines which device has priority in the event of a collision based on the rule of sequential equal priority switching.
Priority Order Method
On the other hand, the priority method determines which device has priority in the event of a collision based on a fixed priority order for each device.
Depending on which method is adopted is determined according to the system’s application and characteristics.
Other Information on VME Boards
1. Bus
The bus mentioned in the above explanation refers to a circuit that transmits data and signals inside a computer. Inside a computer, devices such as CPU, memory, and IO devices are connected by a common bus.
There is an address bus for transmitting memory address signals and a data bus for data transfer between the CPU and memory/IO devices. There is also a control bus for controlling these two buses. The control bus is responsible for controlling the timing of actual input/output by the address bus and data bus.
2. Bus Standard
The initial development standard was a 16-bit bus standard to fit into the Eurocard connector. Later, through expansion, the current VME64 has a 64-bit bus standard and a 32-bit bus standard.