Siemens SIMATIC TIWAY 1 - Manual (Page 27)

OPERATION AND APPLICATION
UNILINK HOST ADAPTER 2–17
USER MANUAL
2.6 SPECIAL NETWORK MANAGER FEATURES
The adapter provides two special features in the MHIU and NM operating
modes. First is the redundant media channels with selectable automatic media
channel switching. Second is the background monitor that will bring devices
online dynamically.
2.6.1 Redundant Media Channels
The adapter provides for two network media channels, Ports 1 and 2, described
in more detail in Chapter 4 . You may select to use only one channel or both by
specifying the appropriate value in the CONFIGURE NETWORK MANAGER
command. Using redundant media increases the mission reliability of the
network or the probability that the network will remain operational when a
random failure occurs. When first initialized, the adapter will communicate on
Channel A or Port 1. Your applications software can then issue the SWITCH
CHANNEL command at any time to change the operating channel for one,
several, or all secondary devices. The channel can be switched even if the auto
redundant media feature is not active; however, if it is active, the Network
Manager function will switch the channel back if a failure occurs. Channel B or
Port 2 must be available or an ERROR RESPONSE will be returned when you
try to issue this command. If the command is accepted, the adapter will return
the command code and the channel code.
The primary function of redundant media is to provide a second
communications path in the event of a failure on the primary path. This can also
be used as a maintenance aid by switching all secondaries to the alternate
channel so that the main channel can be maintained without completely
shutting the network down. Remember, only one media or channel can be
transmitting or receiving in a given instance.
The auto redundant media feature allows the Network Manager function to
automatically switch a secondary device to the opposite channel if
communications can no longer continue on the current channel. The secondary
device will be switched if it fails to respond consecutively in
maximum-retry-count times. The maximum retry count is specified in the
CONFIGURE NETWORK MANAGER command and defaults to 3 times. Once
the secondary has been switched, the Network Manager function will attempt
to communicate again on the new channel up to the maximum retry count. If the
secondary responds, communications will continue on this channel. If the
secondary fails to respond, the secondary will be logged as off-line and all
associated macros will be disabled. All macro response status words will be
updated to indicate that the secondary is off-line, and any network requests
that were pending will return the ERROR RESPONSE.
2.6.2 Background Monitor
Background monitor is a feature of the Network Manager that instructs it to
periodically check the link status of each secondary device. This feature is
active in the Network Manager function in all but the EHA modes and the
SANM mode. The link status information includes the communications status
for each channel and for each secondary, whether the secondary is online or