Appendix 2
Data Transmission
Netiom-UDP uses UDP (User Datagram Protocol) packets to transmit information across the
network. This is a simpler but faster protocol than TCP and more suited to this type of link.
While there is no change in input conditions and no serial port activity a packet is sent at the
end of the user defined update period. The packet is marked as non-urgent so that the remote
end knows that this is a regular update packet and does not need to respond until its own
update period expires. This heartbeat message ensures that the remote end knows that the
link is still in place and that if packets are lost then outputs are still synchronised.
If there is a change of input conditions or a serial port message is received then Netiom-UDP
will immediately transmit a message irrespective of the state of the update period. This
message will be marked as urgent and assigned a message number. On receiving this
message the remote end will acknowledge the packet. The originating end will wait for this
acknowledgement and if it is not received within a fixed period will re-issue it. The re-issues
will continue until either an acknowledgement is received or a new message is generated by
another input change / serial message receipt or until the update period expires.
Each packet is divided into 4 sections. The first section identifies the message as originating
from a Netiom-UDP device. The second section is used for control, marking as urgent / non-
urgent etc. The third section defines the current state of the I/O and is always transmitted.
The final section is only present if there is a serial port message and contains the data
received.
