Telegram SD3
Telegram with fixed information length
SD3/DA/SA/FC/aa/oo/oo/cc/xx/xx/xx/xx/FCS/ED
|<--
L
-->|
is used for sending a query to the recorder.
Where:
SD3
= A2H
Start byte
DA
Target address (bus subscriber address)
SA
Source address
FC
= 15H
Function code
aa
Basic address of the parameter field
oo oo 2 byte parameter address (offset)
cc
Number of data bytes
xx xx
xx xx
4 random bytes
FCS
Test byte (Frame Check Sequence)
Sum of the hex value of the L frame without transfor-
mation for FFH
ED
= 16H
End byte (End Delimiter), code: 16H
L
Number of Bytes in FCS
Transmission rules
The idle state of the line corresponds to the logical “1” signal.
Before data transmission starts − from the PC − a minimum time
of 33 bits (syn-time) as idle state is required for synchronisation.
Pauses of the length
≥
3 frames shall be interpreted as telegram
end.
The recorder inserts a pause of
≤
300 ms each between the
reception of the last stop bit and the first start bit.
Pause between two telegrams:
Telegram
Break
≤
300 ms
Answer telegram
Gaps between two frames:
Frame
≤
0,2 ms
Frame
The receiver checks
−
per frame
Start ,stop and parity bit and
−
per telegram
Start, DA, SA, FCS and end byte.
If the check produces a negative result, the entire telegram
should be rejected as incorrect.
In the reply, the recorder takes over the source address of the
transmitted telegram as target address and inserts its own
address as source address.
8
Data transmission • Parameters
