RCOM NetworksΒΆ

A RCOM network consists of 2 or more users, e.g. control computers or substations etc. One user is always configured as RCOM master. All other users are RCOM slaves.

The users are connected by means of a transmission medium. In case of RCOM, this may be a direct line (point-to-point connection), a dedicated line with multidrop modems (these permit coupling of several users to one line) or a dial-up connection over the public telephone network.

Each subscriber in the network has an address assigned that can be used to address (call) this specific device. This address is a number between 1 and 254 for slaves, or 0 for the master.

The illustration below shows an RCOM network with multidrop modems:

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With each job, the master transfers the specific address of the addressed slave and only this slave will respond to the job.

Master-slave structure

There is a simple convention for controlling data communication on the line: One user in the network is the master. Only the master can send jobs to other users, the slaves. The slaves respond to a job telegram with a telegram that indicates whether the job has been understood and whether it has been possible to execute it.

This always results in the sequence job telegram (response telegram) on the data line.

Addressing in the RCOM network

A complete address has to be specified in order to address a data set on a specific slave. This address consists of the following parts:

  • Number of the slave (NODE). Up to 254 slaves may exist in an RCOM network. Number 0 is used by the master.
  • Number of the data set (ID).
  • Number of data words of the data set to be transmitted (LEN). Data sets do not always have to be transmitted completely. However, a minimum of two data words have to be transmitted and the number of data words has to be even-numbered. Transmission always starts with the first data word.

The illustration below shows an example of a RCOM system and the addressing path:

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Broadcast

Slave number FF hex (255) can be used to transmit specific services to all slaves simultaneously. Such a service is not answered by any of the slaves and is thus repeated several times by the master as a safety measure.

Broadcast is permitted only for jobs where no user data are transmitted (system services).