Specifications
A–9Setting Up the DH-485 Network
Publication
1747-6.2
The following sections explain network considerations and describe
ways to select parameters for optimum network performance
(speed).
Number
of Nodes
The number of nodes on the network directly affects the data transfer
time between nodes. Unnecessary nodes (such as a second
programming terminal that is not being used) slow the data transfer
rate. The maximum number of nodes on the network is 32.
Setting
Node Addresses
The best network performance occurs when node addresses start at 0
and are assigned in sequential order. SLC 500 processors default to
node address 1. The node address is stored in the processor status
file (S:15L). Processors cannot be node 0. Also, initiators such as
personal computers should be assigned the lowest numbered
addresses to minimize the time required to initialize the network.
Setting
Processor Baud Rate
The best network performance occurs at the highest baud rate. All
devices must be at the same baud rate. The baud rate is stored in the
processor status file (S:15H).
Maximum
Node Address Setting
The maximum node address parameter should be set as low as
possible. This minimizes the amount of time used in soliciting
successors when initializing the network. If all nodes are addressed
in sequence from 0, and the maximum node address is equal to the
address of the highest addressed node, the token rotation will
improve by the amount of time required to transmit a solicit
successor packet plus the slot timeout value.
Note that this does not allow any node to be added to the network
without affecting the response time. On the other hand, since the
time required to hold an open station address is greater than the time
required to pass a token, it can be useful to leave a temporary device
(such as a personal computer) connected if there is only one such
device. (A solicit successor packet requires the same transmission
time as the token pass, but there is an added slot timeout period.)
See your programming software user manual or the Hand-Held
Terminal User Manual, Catalog Number 1747-NP002, for the
procedures to set node addresses, processor baud rate, and maximum
node addresses.