Midas Slow Control Bus (MSCB)

by S. Ritt
MSCB Protocol
First performance results


Physics experiments usually have the requirement to control equipment which is commonly refered to as "slow control". This included high voltage power supplies, temperature and similar measurements. For this purpose standards like
GBIB (IEEE-488) and RS-232 are used. The usage of field buses like CAN, Profibus and LON are usually not suitable for small experiments since they require some significant effort to integrate them into an experiment.

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.

July 18th, 2001, S. Ritt