Eltron Astro-System 2 (“AS2”) is a flexible solution to two basic problems when considering astronomical dome automation - dome motion control and air conditioning in it. AS2 consists of several input and output modules, some of which poses a certain degree of local information processing power and autonomy in fulfilling their tasks (i.e. “intelligence”), while others simply gather data and/or rely on commands issued by the central computer for performing actions.
Apart from two general tasks, system software flexibility enables numerous additional input and output functions to be added to it.
All AS2 system components are connected to ELM-4 data bus, which is connected to the central computer (a PC with WindowsTM operating system installed) via the ELM-4 data bus Central Controller module. PC runs the dedicated “AS2 Communicator” ASCOM compatible application software that collects all data and governs the system operation.
Topological structure of AS2 hardware layer is depicted in the following diagram:
ELM-4 central controller performs two-way translation between USB and ELM-4 standard data packages. It also contains galvanic insulation barrier so that overvoltages induced in ELM-4 cables cannot harm the central computer - this is the real problem since astronomical observatories are situated at mountain tops and are consequently exposed to harsh storms and lightning strikes.
In addition, ELM-4 central controller contains a strong beeper and two sets of signaling LEDs. One of LED sets simply indicates ELM-4 data bus health and enables the operator to overlook the system operation and promptly locate any potential problem. AS2 Communicator application controls the other set of LEDs and the beeper by which it can alarm the operator in case an excess situation is detected - this feature is built in having in mind the possibility that PC speakers might be turned off, or that AS2 Communicator application window might be temporarily hidden from the operator’s view etc.
ELM-4 uses standard LAN CAT5 or CAT6 cables for connecting its components, but it uses the conductors differently in comparison to Ethernet specifications:
One pair of conductors is used for data communication at 19.2 kbps rate
Three pairs of conductors are used for supplying electrical power via Central controller module to remote components.
Because of this, connecting ELM-4 components to Ethernet networks directly is prohibited. That is the reason that ELM-4 uses different connectors than standard Ethernet RJ45. Electrical signals used for data transfer are compatible with RS-485 standard. Data rate is intentionally kept low in order to assure higher system reliability and provide for maximum cable length greater than 300m.
ELM-4 compatible components are divided into groups based on their types. Type codes are fixed and encoded during production (in range "1" to "15"). There can be from 0 to 15 components of the same type connected to the same ELM-4 network. Each of them is addressed individually using the combination of the hardcoded type code and arbitrary software defined addresses (also in range "1" to "15").
Data transfer is organized as package communication, with each package being protected from errors by forward error correcting code. ELM-4 system connects to the central computer (PC) via a standard USB v2.0 or v1.1 port using the ELM-4 Central controller module as data format translator.
Access time shorter than 0.3 seconds is guaranteed to each connected component. This is an important attribute as the AS2 system operates in real time and is responsible for the proper operation of very expensive equipment.