As an alternative to those systems, the Midas Slow Control Bus (MSCB) has been developed at PSI by R. Schmidt and S. Ritt . This bus is a kind of "poor man's" version of a field bus with less flexibility, but on the other hand it is optimized for experiment environments and much cheaper (typical 20US$ per node). Although this bus is integrated into the Midas data acquisition system, it can be used independently from Midas.
The MSCB bus uses the RS-485 standard for commication. This standard is similar to the well-known RS-232, but uses differential signals for superior noise immunity. The bus is controlled from a PC using an parallel port to RS-485 converter. On each bus segment, up to 256 nodes can be connected in parallel. Using one layer of repeaters, 65536 nodes can be operated on a single bus, which makes the MSCB bus suitable for large experiments typically found in high energy physics. The bus protocol uses an addressing scheme to talk to individual nodes. Multi-master operation is possible using a token-ring scheme.
The MSCB nodes are based on a new class of microcontrollers which incorporate DACs, ADCs, IO ports and more on a single chip. Currently, the chips from Analog Devices and Cygnal are supported.
Following figure shows a typical layout of the MSCB and a generic node layout:
Several prototype hardware has been built at PSI. Following picture shows a running MSCB system with the submaster at the left side and a couple of nodes:
For more information, follow the links at the top of the page.