Installation manual Reader
24
Modifications, misprints and errors excepted.
English
4.
Connections and displays
4.8.
UART transmission (RS232, RS422, RS485 or similar)
4.8.1.
Bit transmission layer (physical layer)
A full or half-duplex connection such as RS232, RS422 or RS485 is used for the physical layer.
4.8.2.
Data link layer
Transmission is in frames and blocks. A block comprises a maximum of 256 frames. A frame comprises a
maximum of 256 bytes, of which a maximum of 250 bytes can be user data. The result is a maximum block size of
64000 bytes of user data.
The data link layer is used to safeguard the data between sender and recipient. The sender receives a response
from the recipient for each frame received. If the sender does not receive a response from the recipient within a
time window of 350 milliseconds after sending a frame, the frame sent is repeated until the error counter signals
the cancellation of the transmission.
4.8.2.1.
Structure of a frame
5A LL SS FF DD ... DD P1 P2
5A:
Start code for synchronisation
LL:
Number of bytes in the frame not including the start code
SS:
Status byte
FF:
Frame number
DD:
User data
P1:
16-bit checksum low byte
P2:
16-bit checksum high byte
4.8.2.2.
Start code and synchronisation
The start code is used to synchronise the recipient to the sender. It further allows the receiver to synchronise to the
start of a frame when no data have been received for 15 milliseconds.
4.8.2.3.
Status byte
The status byte has the following significance:
50:
Data packet
A0:
Response: “OK”
A1:
Response: “Memory error” (the receiver was unable to allocate any memory for the data block received)
A response is only 3 bytes long and is not CRC checked.
“OK” response: 5A 02 A0
“Memory error” response: 5A 02 A1
4.
Connections and displays
