Project Planning Manual SIPART DR20
3.2.7 Bus Interface
Together with the module 6DR2803-8A/8C the SIPART DR20 can transmit and receive operating
states, process variables, parameters and configuring switch positions via a serial interface. Up to
32 controllers with these interface modules can be connected in parallel to a bus. Combination with
TELEPERM D devices is also permissible if the control of the higher-level computer permits this.
Data transmission is at a rate between 300 and 9600 bit/s set using configuring switch S42 and in
semi-duplex mode with asynchronous transmission of ASCII characters in a 10-bit frame (start bit,
ASCII character with 7 bits, parity bit and stop bit). The error message character NAK is
transmitted in full-duplex mode so that messages can be repeated rapidly in the event of a fault.
The controller is passive and only reacts when requested. The complete bus must be controlled by
the higher-level system (master/slave operation).
The configuring switches S42 to S48 are used to define the response of the serial interface and the
station numbers in the bus (between 0 and 31). These configuring switches can only be set
manually on the device and not via the interface itself.
The transmission procedure (according to DIN 66258 A or B) commences with the start character
STX, followed by the station number, the data, the end character ETX and, if applicable, a cross-
check sum for additional data protection. The quantity of data to be transmitted, the start address
of a list range and a code for transmitting or receiving data are first defined in the data to be sent to
a controller. These are then followed, if applicable, by the data to be received. If the message is
received without faults, the SIPART DR20 replies immediately with a message with exactly the
same format: STX, station number, data, ETX and, if applicable, cross-check sum. The data
section is omitted because no values are requested.
A particularly short procedure has been defined in addition in order to rapidly scan alarms and
other status signals from all stations connected to the bus:
Request: STX, station number with code bit, ETX, cross-check sum
Reply: STX, station number with reset bit, the two status bytes STN and STA,
ETX, cross-checksum
The serial interface is activated by configuring switch S42:
S42 = 0 The controller transmits all variables, i.e. parameters, configuring switches,
process variables and status signals, but does not receive any variables
S42 = 1 The controller transmits all variables, but only receives parameters and the
configuring switch positions S1 to S41.
S42 = 2 The controller transmits all variables, and receives parameters, the configuring
positions S1 to S41 as well as wES, yES, STes (CB, N, Si, BL and BAes).
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