Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

198 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Note that in the following procedures, no new procedure is commenced until the
previous procedure is terminated. It is for this reason that a single-bit frame count bit is
sufficient for protection against frame sequence number errors; there is a frame window of
exactly one.
SEND/NO REPLY procedures
The frame is transmitted, and a minimum line-idle time of 33 bit-times is required before
any further transmissions by the primary. On receipt of the message at the secondary, it is
checked for error by comparison with the checksum octet, and if valid it is notified to the
service user.
SEND/CONFIRM procedures
The primary station will transmit the message. If a confirmation is not received from the
secondary station within a configured time-out period, it will re-transmit the message up
until a configured number of retries.
If the secondary station receives the message, it will respond with either a positive or
negative confirmation, function code 0 or 1. FC=0 means the message is correctly received
and accepted, and in this case the procedure terminates. FC=1 means the secondary cannot
accept the message, because its buffer is full or some other reason. In this case the primary
will retry, up until a configured number of retries.
This procedure makes use of the frame count bit (FCB) to ensure that the message
sequence is not disturbed. The FCB is toggled with each SEND/CONFIRM transmission
from the primary station, and an expected FCB flag is maintained by the secondary
station. If a message sent by the primary is not confirmed by the secondary, it is retransmitted
with the FCB unchanged. Thus a message lost or corrupted in the primary to
secondary transmission direction would when retransmitted still have the expected FCB
value. Alternatively, if the problem was that the confirmation from the secondary station
was lost or corrupted, the secondary station would be able to recognize receipt of a
retransmitted message from the primary by the unchanged FCB bit. In this case it retransmits
the original confirmation message.
As an alternative to sending a confirmation frame, which is a minimum of six octets in
length (that is if single-octet address is used), the single control character response (hexadecimal
0xE5) is allowed. This option may be used to improve transmission efficiency
when there is no need to transmit any other information back to the primary station.
REQUEST/RESPOND procedures
This procedure is similar to the SEND/CONFIRM except instead of receiving a confirmation
back from the secondary station, a frame containing data is returned, or a negative
response is returned indicating that no data is available. In the case of a negative response,
either a frame with FC=9, or the single control character (0xE5) may be returned.
As for the SEND/CONFIRM procedures, the frame count bit is toggled at each end for
each message transmission, and this is used to detect errors in the transmission in either
direction. Basically, when both primary and secondary have incremented their frame count
bits and they agree, the procedure is complete.
Philosophy of transmission
For unbalanced transmission only the controlling or primary station on a link can initiate
transmissions. Because of this it is necessary that a polling system is implemented by the
primary station in order to determine if there is change data available at each secondary
station.