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Implemented protocols in UNIGATE RS
Deutschmann Automation GmbH & Co. KG
28
UNIGATE fieldbus gateway for ProfibusDP V. 4.7
29.4.11
In addition, a "Response time" which corresponds to the maximum time up to which the field-
bus-master responds after a request must be set an dat the rotary switch on the RS-side the
MODBUS-ID has to be set, under which the gateway is addressed in the Modbus.
Since the Modbus operates with a variable data format - dependent on the required function and
data length - but since the fieldbus requires a fixed data length, this must be preset by means of
a selection in the GSD file (input and output are identical). This length should be selected by the
user such that the longest Modbus request resp. response can be processed. If a fieldbus-tele-
gram is longer than the preset fieldbus length, the gateway signals an "Rx buffer overflow“.
9.5.3.2
Data structure
9.5.3.3
Communication sequence
The gateway always acts as the Slave with respect to the fieldbus and also acts as Slave at the
Modbus end. A data exchange is always initiated by the MODBUS-Master via the RS-interface. If
the Modbus-address (1st Byte) which is sent out by the Modbus-Master is identical with the
address set on the gateway, the gateway sends the received data (without Modbus-address and
CRC-check sum) to the fieldbus-master (look picture above). With it the gateway optionally com-
pletes as an introduction a Trigger byte and a Length byte.
The fieldbus-master detects when it has to analyse a record via the Trigger byte which is incre-
mented by the gateway at every inquiry. The number of the following Modbus-data is to be found
in the length byte.
Now the fieldbus-master has to analyse the Modbus-inquiry and it has to send back the answer
in the same format (optionally with the leading Trigger byte and Length byte) via the fieldbus to
the gateway.
The gateway then takes this answer and completes the Modbus-address and the CRC and
sends the data to the Modbus-Master via the RS-interface. With it the data exchange is com-
pleted and the gateway waits for a new inquiry from the Modbus-Master.
9.5.3.4
Status report
If no response occurs within the stipulated "Response time" from the fieldbus-master, the gate-
way a "Time-out-Error".
The last Modbus-ID that was transferred to the RS-interface is indicated by the yellow LEDs.
9.5.4
UNIGATE as Modbus-ASCII Master
-> For the description see chapter 9.5.2 "UNIGATE as MODBUS-Master"
Содержание UNIGATE RS232/422
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