22. Serial Protocols
ROX™ v2.2 User Guide
220
RuggedBackbone™ RX1500
22.1.3. Serial Protocols Concepts And Issues
22.1.3.1. Host And Remote Roles
The RX1500 can either initiate or accept a TCP connection for serial encapsulation. It can establish
a connection from field (“remote”) equipment to the central site (“host”) equipment, vice versa, or bi-
directionally.
Configure the RX1500 at the host end to connect to the remote when:
• The host end uses a port redirector that must make the connection.
• The host end is only occasionally activated and will make the connection when it becomes active.
• 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.
22.1.3.2. Use Of Port Redirectors
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.
22.1.3.3. Message Packetization
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:
• Packetize on Specific Character,
• Packetize 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.
If configured to packetize on a timeout, the server will wait for a configurable time after receiving a
character before packetizing and forwarding. If another character arrives during the waiting interval,
the timer is restarted. This method allows characters transmitted as part of an entire message to be
forwarded to network in a single packet, when the timer expires after receiving the very last character of
the message. This is usually the only packetizer selected when supporting ModBus communications.
Finally, the server will always packetize and forward on a full packet, i.e. when the number of characters
fills its communications buffer (1024 bytes).
22.1.3.4. Use of Turnaround Delays
Some RTU protocols (such as ModBus) use the concept of a turnaround delay. When the host sends
a message (such as a broadcast) that does not invoke an RTU response, it waits a turnaround delay
