Rockwell Automation 1747-L40 SLC 500 Fixed Hardware Style - Manual (Page 113)

Appendix A
Setting Up the DH-485 Network
A–8
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.
If some nodes are connected on a temporary basis, do not assign addresses to
them. Simply create nodes as needed and delete them when they are no
longer required.
Setting Processor Baud Rate
The best network performance occurs at the highest baud rate, which is
19200. All devices must be at the same baud rate. The default baud rate for
SLC 500 devices is 19200. 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 the Hand–Held Terminal User Manual, Catalog Number 1747–NP002,
or the Advanced Programming Software User Manual, Catalog Number
1747–NM002, for the procedures to set node addresses, processor baud rate,
and maximum node addresses.
Important: The SLC 500 Series A (only) processors set the maximum node
address to 31 when power is cycled increasing initialization and
response time of the network.