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RSTi-OM User Manual
GFK-3212A
12.3.9
ISDU Read/Write
Since MQTT lacks intrinsic support for request/response semantics, ISDU read/write requests and responses
are handled via a pair of topics:
Requests for ISDU read/write are published by other clients to the "request" topic shown above. The
client_transaction_id is an arbitrary string chosen by the requesting client and should be chosen to be unique.
After the ISDU operation is completed, the IO-Link Master will publish the response to the corresponding
"response" topic (with the same client_transaction_id as the request).
12.4
ISDU Request Payload
The ISDU request payload is a JSON object with the fields described below.
Name
op
index
subindex
Fields specific to read requests:
format
Fields specific to write requests (exactly one of uint, raw, or str must be present)
raw
uint
str
len
In a written request with str data and a len field, the string will be NULL-padded to the requested length
before being written to the device.
MQTT Interface
"port": 2,
"value": 0
}
•
MqttTopicBase/port/n/isdu/request/client_transaction_id
MqttTopicBase/port/n/isdu/response/client_transaction_id
•
Type
string
integer
integer
string
array
integer
string
integer
Description
Required — must be "read" or "write"
Required
Optional (defaults to 0 if not provided)
Optional — if present, it determines the format of the returned
read data in the response. Should be one of "str" "raw" "uint". If
not provided, read data will be returned in all formats.
The array of integer byte values (decimal)
Integer data value (requires len field)
UTF-8 data string (len field is optional)
Required for uint data, optional for str data. Controls the number
of data bytes written.
Section 12
Jun 2021
122
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