Xylem Expert 7060_7070_Modbus - Manual (Page 33)

Communication protocol Serie30
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3 Data-link layer
This section describes how data interchange is effected on this bus. The data and their check and control structures are grouped
together to form messages. These constitute the smallest communication unit, i.e. only messages can be exchanged between
the devices. As a half-duplex protocol is in use here, only one device can use the bus as a transmitter at any one time. All other
devices are then in receiver mode. The master takes the form of a PC or microcontroller, for example, and the devices are the
slaves. Each message exchange takes place under the control of the master. The message contains the address for the
receiving slave.
This results in the following 2 options for data interchange :
a) Broadcasting
This mode of communication enables the master to transmit a message to all slaves
simultaneously. The master does not receive a reply, however, and is thus unable to check whether
the message has been correctly received by every slave.
b) Data interchange This mode of communication enables the master to communicate with a single slave. This normally
involves the transmission of two messages: the master transmits a request and the slave responds
to this request. Only the master is permitted to request a response. The request is received by every
slave, but only the selected slave responds. The response must be received within a stipulated time,
otherwise the master will assess the attempt as failed and must transmit the request again.
3.1 Transmission format for the serial interface
The data are transmitted serially via the bus. The following format applies:
• 1 start bit
• 8 data bits (the least significant bit first)
• 1 stop bit
• no parity (default)
• 9600 baud (default) or 115’200 Baud
This results in 10 bits (11 bits with active parity bit) per transmission byte.