Abstract: A processor arrangement is described having a programmer logic controller (PLC) operating to a ladder diagram process. A number of event inputs (11-15) are provided connected to an interrupt buffer (31), in turn connected to a real time clock (32), for recording times of events of signals received on the event inputs and generating interrupts. Sequential storage locations in a cyclical buffer (60) store interrupts and their event times, where events occurring within a predetermined incremental time window are stored, with their event times, in a storage location for that time window. Each location of the cyclical buffer is inspected in sequence upon each new step of the sequential process. The PLC is arranged to operate on any events stored in the buffer which are affected by the new step of the sequential process before proceeding to the next step of the sequential process.

Abstract: A communications system is described having a number of communicating units arranged for communication over a common communications channel. A first communicating unit (10) transmits a data transmission (104) followed by an end-of-data indication (102) and a predetermined period of further transmission activity (101) to a second communicating unit (12). During the period of further transmission activity (101) of the first unit, the second unit seizes the channel (103) and transmits a preamble (105) prior to its response data. This process continue ensuring that periods of further transmission activity (101) and the preamble (105) keep the channel open for the duration of the communication, for sole use by the communicating units. The invention is applicable to, but not limited to use in supervisory control and acquisition-of-data (SCADA) systems.

Abstract: A data communications system comprising first (B), second (C) and third (A) remote units arranged for communication over a common communications channel. A first message to be sent to the second unit (C) is generated at the first unit (B). The first unit (B) waits for the channel to become available for independent transmission of the message. If (t.sub.0), before the channel becomes available for independent transmission, a new message is received from the third unit (A) requiring an acknowledgement, the new message is received from the third unit (A), an acknowledgement of receipt of the message is transmitted (t.sub.1) and in a substantially continuous transmission (t.sub.2), the first message to the second unit is transmitted in a "piggy-back" manner. In a preferred arrangement, multiple messages can be piggy-backed to different units. Priorities can be assigned to these messages for acknowledgements.

Abstract: A method of communications between a master unit and a slave unit of a supervisory control and data acquisition system is provided. The communications protocol has at least two bytes of data where the first byte (100) is an address byte containing address information for addressing the slave unit and the second byte (101) is a control byte containing an expansion bit and two address/type variable bits to be interpreted by the slave according to a rule set. In the rule set, the meaning of certain control bits (107) is related to the state of an expansion bit (103). The expanion bit defines the presence or absence of a third byte (102).

Abstract: This invention relates to communications apparatus, such as remote data/control apparatus. A first unit (10) and a second unit (11) are arranged to exchange messages over a communication channel. To avoid collision between responses between different remote units, the first unit has device for requesting a response from the second unit and circuit for communicating to the second unit a value (23) representing a time delay and the second unit has device for monitoring the channel, determining when the channel becomes free and transmitting its response to the first unit delayed by that time delay after the channel becomes free.

Abstract: This invention relates to a supervisory control and data acquisition (SCADA) system in which a central station (10) sends control information to, and acquires information from remote stations (11, 12) over a wire, radio or other link. Each station comprises a tabular memory (15-18) comprising storage locations in an `x` direction for storing data relating to different elements of the second system and storage locations in a `y` direction for storing data relating to different parameters of those elements. Transmitting means are provided for transmitting a message from one unit to the other unit, said message comprising an `x` part, a `y` part and a qualifier, for reading from and writing to storage locations in the tabular memory of the other unit depending on the result of a logical operation defined by the qualifier. The tabular memory can equally be considered to have a tree-like structure.