Promag TR515 User Manual page 12

Select designated terminal ('D', 44H)
Command DNN
Reply ANN,TR515
NN is a machine (terminal) number in Hex form (00H-FFH), 00H is a
"universal" number (see below for details).
'D' is a special command. It is used to select a particular TR515 terminal.
All terminals power up in a "deselected" state. Each terminal ignores all
incoming commands until it receives the D-command whose machine number is
either equal to the machine number of this terminal, or 00. After that, the
terminal switches into "selected" state and responds to all subsequent
commands until new D-command with a different machine number is issued.
Thus, the D-command should always be used to initiate communications
between the host and the designated TR515.
Note: the TR515 never returns any error codes in reply to this command.
This holds true for both "Illegal command" error and "Command failed" error.
The former is not responded to, because when there is some discrepancy in the
format of the D-command issued by the Host (which can only be in the Machine
Number field), no particular terminal can be "sure" that it is being addressed.
Therefore, no terminal on the network assumes responsibility for sending a
reply. "Command failed" situation can only arise because of incorrect machine
number setting value in the TR515 memory. In this case, the terminal will be
unable to compare its internal machine number with the one supplied in the D-
command. Hence, the Terminal will not be able to make sure that it is being
addressed.
Since 00 is a universal machine number, it works with any terminal. Use it
only when you have a single terminal connected to the host. Universal machine
number also comes handy during initial terminal setup (to assign a unique
machine number to the terminal prior to placing it on a multi-terminal network).
The terminal replies with its actual machine number even if you address it
using 00. The only exception is when the terminal is unable to retrieve its own
machine number due to some internal malfunction. Reply will contain 00 instead
of an actual machine number in this case.
TR515 User's Manual
10
Possible error codes: none