SCHRACK TECHNIK GMBH 2
1.
2.
On the same phisical line only instruments can attached (master included).
Up 255 different instruments can managed the protocol.
See the description the data packet the next paragraphs for more details.
A delay between the response from the slave and the next command could necessary and is
specified for each device (timing).0 DATA MESSAGE DESCRIPTION
The generic data message composed following:
Device address Functional code Data CRC word
Two answers are possible :
Answer containing data
Device address Functional code Data CRC word
Error answer
Device address Functional code
+ 0x80
Error code CRC word
. order increase
the number the slave instrument, the necessary repeaters must used.0 ABSTRACT
Phisical level
The phisical communication line complies with the EIA-RS485 standard half-duplex modality.
The communication parameters are:
Baud rate programmable (device dependant)
bit 8
stop bit 1
parity programmable (device dependant)
Data link level
The data are transmitted packet form (message) and are checked word (CRC).
There are limitations the number possible retries done the master.
In this case, only two wires are used, only one instrument time can engage the line; this means
that there must master which polls the slave instruments the demand and the request are
alternated.
Application level
The communication protocol used MODBUS JBUS compatible
