EUCHNER CIS3A-Mini - Manual (Page 10)

EUCHNER Identsystem CIS3A-Mini
084727-01-02/03 Subject to technical modification Page 10 / 22
3.2.2.2 Control receives [1]
If the control receives control character STX from the peripheral in idle state, it responds with
DLE. If the control receives another character (apart from STX) in idle state, it waits for the
character delay type (ZVZ, typically: 100 ms) to elapse and then sends the control character
NAK. After each character, the next character is awaited during the character delay time
(ZVZ). If the character delay time elapses without reception, control character NAK is sent to
the peripheral.
If the control detects character string DLE ETX BCC, it terminates reception. It compares
the received block check character BCC with the internally generated longitudinal parity. If
the block check character is correct and no other reception error has occurred, the control
sends control character DLE. If the BCC is incorrect, control character NAK is sent to the
peripheral. A retry is then awaited. If it is not possible to receive the block error-free even
after a total of 6 attempts (specification of the 3964R protocol), or if the retry is not started by
the peripheral within the block waiting time of 4 seconds, the control aborts reception.
If transmission errors (lost character, frame error, parity error) occur during reception,
reception continues through to connection clear down and control character NAK is then sent
to the peripheral. A retry in the manner described above is then awaited.
3.2.3 Summary of the most important points
· DLE doubling:
In order for the control to be able to distinguish between control character DLE and any
randomly occurring DLE as user information character, a further DLE must be sent in
the case of a DLE as user information character. This means that if a byte with ASCII
value DLE ( 10
hex
) occurs within the telegram core, this character must be transmitted
again so that it is not interpreted by the distant station as a control character for
connection clear down.
· Block check character (BCC):
A block check character is sent at the end of each data block for data integrity. The block
check character BCC is the even longitudinal parity (exoring of all data bytes) of a
transmitted or received block. Generation starts with the first user data byte (first byte
of the telegram) after connection set-up and ends after characters DLE and ETX on
connection clear down.
· Retry attempts in the case of errors:
If an error occurs for any reason during data transfer, a total of 6 attempts are made in
order to transfer the data correctly.