ProSoft Technology PS-QS-1x10-0781 - Manual (Page 64)

FieldServer Configuration Manual Page 64 of 90
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10.3.1 Single Port Server:
Most devices (information Servers) that wait to be polled for data are only capable of communicating with one
Master device through one port at any time. The method employed by FieldServer to achieve redundancy with
these devices is depicted in Figure XIII.
Figure XIII- Single Port Server
The RS-232 ports of the two FieldServers are routed via an RS-232 splitter device allowing them to be connected to
the single RS-232 port of the PLC. These splitter devices are robust and manufactured using just a few passive
electronic devices (diodes), to minimize the chances of failure. In this arrangement the two FieldServers have to
poll the Server alternately to prevent serial message collisions. A token passing method is used to achieve this.
The FieldServer with the token is allowed to poll the Server on its serial port. When the response is received, the
token is transferred over the Ethernet to the other FieldServer which then takes its turn. Timeouts, lost or
duplicate tokens or failed FieldServers are also managed in an elegant fashion. Clearly, data would be lost if the
Single Port Server were to fail in any way.
10.3.2 Dual Port Server:
Server devices that make two ports available for redundant communications are referred to as Dual Port Servers.
Each port can be connected to a separate FieldServer; allowing each FieldServer to poll for data at will. This
arrangement is depicted in Figure XIV. Should one of the ports fail then the data is routed to the Client through
the PEX Tier of the other FieldServer and then through the SCADA tier to the Client (Refer to Figure XIV and Section
10.3.2). If both FieldServers were to fail, obviously the data cannot get to the Client at all. This means that there
ARE situations where two points of failure can occur and cause the system as a whole to fail. With dual redundant
systems this is unavoidable.
ETHERNET
HUB1
HUB2
Ethernet
10/100 Base T
Serial Port
Active FieldServer
Passive FieldServer
PLC
Ethernet
10/100 Base T
Ribbon cables
(FS-8915-16),
transfer hardwired
ports to standby
FieldServer