Safety System

Application access

Application access may be granted from a system administrator in the Controls Department.

Access to Safety System applications is achieved by asking adeshtux to forward an XWindow to your local computer via ssh. This is handled through a single user, mcr. The mcr user has restricted access via .k5login. The mcr does not get a shell. A bash script is run on login that first runs kdestroy to prevent any tickets being shared on that user. The script then matches on the keyword set page, history, radmon, and apeman. If the argument doesn't match any of these cases it will tell the user what is acceptable and exit. That same keyword set prepended with /esh/bin/ is also acceptable to prevent breaking any existing systems.

Hardware Communication

Colloquially we refer to the field bus as RS485 but this can be confusing for someone investigating the technical implementation of this system. By convention this system behaves like an RS422 bus.

RS422: Each Bus only offers One-way communication. A single transmitting (master) device to one or more receiving (slave) devices on a single pair of wires. Two-way communication requires TWO RS422 buses in parallel. One for each direction. Hence a two-way RS422 bus requires 4 wires1.

RS485: Each Bus offers Two-way communication. Multiple devices can share a single pair of wires. Each device has a transceiver allowing both transmitting and receiving capabilities. This allows point to point two way communication between any given device anywhere on the bus to any OTHER device anywhere on the RS485 bus. Hence two-way communication requires, at a minimum, ONE RS485 bus1.