Chapter 19 – Configuring Serial Protocols
The host will sequentially poll each RTU. Each poll received by the host server is
forwarded (i.e. broadcast) to all of the remote servers. All RTUs will receive the
request and the appropriate RTU will issue a reply. The reply is returned to the host
server, where it is forwarded to the host.
Serial Protocols Concep ts And Issues
Host And Remote Rol es
RuggedRouter either places a TCP connection or accepts one. The connection can be
made from the field or “remote” equipment to the central site or “host” equipment,
vice versa or bi-directionally.
Connect from the host to the remote if:
•The hos t end u ses a p ort redi rector that mu st mak e the c onnectio n.
•The hos t end is only o ccasion ally ac tivated and wi ll make the c onnectio n
when it become s activ e.
•A host end firewall requires the connection to be made outbound.
Connect from the remote to the host if the host end accepts multiple connections
from remote ends in order to implement broadcast polling.
Connect from each side to other if both sides support this functionality.
Use Of Port Redire ctors
Port redirectors are PC packages that emulate the existence of communications ports.
The redirector software creates and makes available these “virtual” COM ports,
providing access to the network via a TCP connection.
When a software package uses one of the virtual COM ports, a TCP connection is
placed to a remote IP address and TCP port that has been programmed into the
redirector. Some redirectors also offer the ability to receive connections.
Message Packetizat ion
The server buffers received characters into packets in order to improve network
efficiency and demarcate messages.
The server uses three methods to decide when to packetize and forward the buffered
characters to the network:
•Packeti ze on Specific Chara cter,
•Packeti ze on timeout and
•Packetize on full packet.
If configured to packetize on a specific character, the server will examine each
received character and will packetize and forward upon receiving the specific
character. The character is usually a <CR> or an <LF> character but may be any
ASCII character.
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