Modular crate electronics
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Modular crate electronics are a general type of electronics and support infrastructure commonly used for trigger electronics and data acquisition in elementary particle physics experiments. These types of electronics are common in such experiments because all the electronic pathways are made by discrete physical cables connecting together logic blocks on the fronts of modules. This allows circuits to be designed, built, tested, and deployed very quickly (in days or weeks) as an experiment is being put together. Then the modules can all be removed and used again when the experiment is done.
A crate is box (chassis) that mounts in an electronics rack with an opening in the front facing the user. There are rails on the top and bottom of the crate that extend from the open (user) end to the back end of the crate. The back end of the crate contains power and data connectors that modules connect to. Electronics modules slide into the crate along the rails and plug into the power/data connectors at the back. Modules have signal connectors, controls, and lights on their faceplate that are used to interact with other modules.
Some modules just draw power from the backplane connectors and have all of their data inputs and outputs on the front plate. Other modules take inputs or controls to and from the backplane or have their behavior controlled from the backplane. Some types of modules have active circuitry inside them, and act almost as small computers; others are not stateful at all and are only dumb single components.
Types of Crate Systems
There are four main types of modular crate electronic systems used on particle physics experiments.
The earliest and simplest crate module standard is the NIM (Nuclear Instrumentation Module) standard. A NIM crate only has power on the backplane, there is no data bus or data connectors. The NIM backplane connector is an irregular arrangement of individual pins into sockets in the crate. NIM modules typically have multiple single logic blocks on the front with both inputs and outputs on the front panel. A typical NIM module might be, say, four discriminators on the front panel, or three AND gates. NIM modules can be hot swapped, since there are no data connectors at the back.
A later crate standard is Computer Automated Measurement and Control, or CAMAC. CAMAC modules are much thinner than NIM modules. The backplane connector of a CAMAC module is a card-edge connector; because of the possibilities of mis-aligning the connectors upon plugin, CAMAC modules are NOT hot swappable. The CAMAC backplane contains a signally protocol for the crate controller to set the values of registers in modules (for configuration) and to read values of registers (for data acquisition). Due to the slowness of the data communication along the backplane, once FASTBUS was invented, CAMAC modules were mostly used for modules that needed to be computer-configured but not for data acquisition.
FASTBUS was a crate/module standard developed later than the other two for high-speed parallel data acquisition. Rather than individual components, FASTBUS modules tended to be data acquisition modules with lots of input connectors on the front, and then the stored data would be read out on the backplane. The connectors on the back of a FASTBUS module were 2 parallel pin sockets on the module and pins sticking out of the backplane. The main connector in a fastbus crate covered about the bottom 2/3 of the module. There was also an upper connector that was merely pass-through pins to the back side of the backplane; this allowed custom modules to be plugged in there.
Fastbus modules are much taller than the other types of crate modules, so the crates are correspondingly taller.
The FASTBUS backplane was a full data bus where any module could negotiate to be master of the bus to send or receive data.
VME (VMEbus) was a bus originally designed to provide an expansion bus for the Motorolla 68000 series processor, but it also became a module electronics crate standard. The first editions of VME were three pins wide with pin sockets on the modules and pins on the backplane. In later editions of the physical standard expanded the connectors with two more rows of pins/sockets on the edges for grounding.
VME was mostly designed as a computer bus, so its modules were largely data acquisitioni modules, not modular electronics.