An interface allows for the transfer of real time control data with
guaranteed delivery times between devices on a general purpose network and
an industrial control system. A proxy server takes the role of a TCP/IP
router and is configured to control the rate at which messages are
forwarded from the non-real time to the real time portion of the network,
keeping the loading of the real time portion stable regardless of the
external non-real-time communication demand. Real time data is
preconfigured and exchanged in a regular sequence, with the cyclic update
period known. The length of any communication message necessary to
transmit this information, plus any sequencing and acknowledgment
overhead, is calculated in advance and aggregated. Each device with
authority to transmit on the shared medium is given a time budget,
calculated so that the total transmission quantity in unit time is
deliberately limited to some fraction of the maximum transmission
capability of the network. All communication from devices whose traffic
loadings cannot be so controlled is arranged to pass through the proxy
device in order to gain access to the deterministic network, and the proxy
enforces the budget limits by introducing deliberate delays to the request
messages if necessary.
