Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 194)

Publication AG-UM008C-EN-P - February 2005
5-8 Configuring SLC 500 Processors with 1747-KE Interface Modules
Parameter Selections
Duplicate Packet Detection Duplicate packet detection lets the interface module detect if it has received a message that is a
duplicate of its most recent message from the master station. If you choose duplicate packet detection,
the processor will acknowledge (ACK) the message but will not act on it since it has already performed
the message’s task when it received the command from the first message.
If you want to detect duplicate packets and discard them, choose Enable.
If you want to accept duplicate packets, choose Disable.
Checksum With this selection, you choose how the interface module checks the accuracy of each DF1 packet
transmission.
BCC: This algorithm provides a medium level of data security.
It cannot detect:
•
transposition of bytes during transmission of a packet
•
the insertion or deletion of data values of zero within a packet
CRC: This algorithm provides a higher level of data security.
Select a method that all your devices on the network can use.
When possible, choose CRC.
Constant Carrier Detect If you want the interface module to monitor the carrier from the modem (DCD signal), choose Enabled.
The module will not begin communication until the carrier is detected.
If the remote modem does not normally receive a constant carrier from the master modem, choose
Disabled.
Modem Init String Enter an ASCII string to configure your modem by using Hayes commands upon every power cycle of
the interface module.
Hardware Handshaking Choose enabled to use the RTS and CTS signals for controlling the modem.
Poll Timeout The timer keeps track of how often the station is polled. If the station has a message to send, it starts a
timer. If the timer expires before the message is sent, then the error bit is set on the MSG instruction.
Message Retries The number of times the processor will resend its message to the master station if the processor does
not receive an acknowledgment.
RTS On Delay RTS on delay is the amount of time, in 5 millisecond increments, that elapses between the assertion of
the RTS signal and the beginning of the message transmission. This time allows the modem to prepare
to transmit the message.
RTS Off Delay RTS off delay is the amount of time, in 5 millisecond increments, that elapses between the end of the
message transmission and the de-assertion of the RTS signal. This time delay is a buffer to make sure
that the modem has transmitted the message, but should normally be left at zero.
Local/Remote Mode Local mode requires a master station that is capable of specifying both a station address and a
destination address. Because the interface module acts as a slave on a half-duplex network, the
half-duplex master’s access to the DH-485 node is indirect. The destination address and the station
address are generally different.
In Remote mode, the module appears transparent to the half-duplex master so that the remote
SLC 500s can be polled directly as individual slaves on the half-duplex network. The interface module
responds to the half-duplex master if the station address specified corresponds to the node address of
any (token-passing) station on the DH-485 network connected to that interface module.
Remote mode is preferred as it allows remote programming of all SLC 500 processors, as well as polled
report-by-exception messages from SLC 5/02 processors on DH-485 to the master station.