Specifications

80
Schneider Electric
Schneider Specifications (continued)
b The time between two characters in a frame must always be less than three
characters (1 character = start bit, data bits, parity and stop bits).
b The time between two frames (master or slave) must always be greater than or
equal to three characters.
b If the device is programmed with parity, verify the parity of all bytes in a frame
received.
b If the device is programmed with no parity, refuse frames that have bytes with
parity.
b The following controls should be performed on a master or slave frame:
v CRC 16 correct,
- correct slave number;
- correct function code and sub-codes (if any),
- correct field length (limits authorised by the protocol),
- actual length correct (length field = number of bytes received),
v a master should:
- signal errors (error message, screen, LED, etc.) from slave responses,
- always be able to communicate after an incorrect response from a slave,
- refuse a slave response than arrives during transmission of a master frame (full
duplex not authorised by Jbus),
- verify that the structure of a frame transmitted conforms to the protocol (number of
variables, number of bytes £ 255, etc.).
Notes:
b A character error indicates an error in the format (framing), framing, parity or
overrun.
b An incorrect frame length is detected by verifying the "number of data" field (if any),
the "number of bytes" field (if any) and the actual length of the frame.
b For a frame longer than 255 bytes, the slave must wait for the end of the frame
(detection of a 3-character silence). The frame is not processed and counter 2
(frame with CRC error) is incremented only once.
b There is now a new diagnostic counter 6 for function 13 (program download). The
old counter 6 "PLC not ready" is now counter 7. The new counter 6 is incremented
each time a "no acknowledgement" exception response is transmitted (including
broadcast). In remote download mode all exception codes (1, 2, 3, 4, 8 and 9) are
replaced by the "no acknowledgement" code (7). This is the case whenever function
code 13 has been recognised (see the <function code unknown> -- no algorithm).
Function 14 can be used to obtain more detailed information on errors.
Guide to Integration
Modbus / Jbus