User manual
IEC61131 User and Reference Manual
April 22, 2008
412
Table Size
The translation table holds 128 translation entries. This is sufficient to re-transmit one-half of
256 possible addresses. On a single port controller only 128 translations are required since
each address must translate to a different address for re-transmission on the same port see
Invalid Translations.
Invalid Translations
The following translations are not valid. The described action is taken when these
translations are encountered.
Re-transmission on the same port with the same address is not valid, except for
broadcast messages. This restriction is required because many message responses are
identical to the command. It is impossible for the master station to distinguish between
the re-transmitted message and the response from the slave. The re-transmitted
message would appear to be the response.
The protocol re-transmits broadcast messages on the same port. Some stations will
receive the broadcast message twice. The master station will also receive the message
and may execute it if it is able to operate as a slave. The user must bear these
consequences in mind when forwarding broadcast messages.
The store and forward controller also processes broadcast messages.
Translations where either of the station addresses are the same as the controller station
address for the port, are not valid. The protocol processes these messages as if they
were directed to the controller. It does not look up the address in the translation table.
Translations with non-existent port numbers or invalid addresses are not valid.
Multiple translations for a port and station address combination are not valid.
Translations where one station is DISABLED and the other station is not, are not valid. A
DISABLED translation is a valid translation.
Store and Forward Configuration
The Store and Forward configuration varies depending on the controller you are configuring.
The configuration for each type of controller is described in the following sections.