Local communication networks for industrial automation

This paper presents a study of the physical and data link layers of the dynamic token-passing bus protocol. This protocol is a variant of the IEEE 802.4 token-passing bus protocol and can therefore be applied to communication systems which support industrial automation. Simulation results have been used for operational validation and extensive evaluation of the communication network's performance. The complex process of allocating a portion of the channel bandwidth to scheduling control, in a completely distributed fashion, has been investigated. The dynamic token-passing scheme exhibits random channel assignment for low bus utilization and token-assigned behaviour for heavy utilizations. Therefore, the protocol is capable of efficient operation for the full range of channel utilization, while its dual-phase behaviour provides for a reliable and flexible channel assignment scheme. The efficient range of each phase is configuration-dependent. Thresholds have been determined for switching between the modes, and a hardware unit has been proposed which monitors the bus activities and, using existing signals, generates the switch-over command. Two recommendations for modification of the existing 802.4 standard are also proposed which increase the reliability and reduce the overheads of the logical ring maintenance functions.