Rockwell Automation 2706-MB1 MESSAGEVIEW 421 SOFTWARE MANUAL - Manual (Page 77)

4–29 Planning an Application
Publication 2706-817
Hardware handshake for an auxiliary port
A MessageView terminal may have its RS-232 port connected to an
auxiliary device. A number of auxiliary devices are listed on Pages
5–6 and 5–7.
The RS-232 cable has an RTS line from the terminal to notify the
device when it is ready to receive, and a CTS line from the device to
notify the terminal when it is ready to receive. These are used as
follows:
•
If the terminal’s input buffer is full, the terminal asserts the RTS
line so the auxiliary device will stop sending data.
•
If the terminal detects an asserted CTS line, it will not send any
more data.
•
If the terminal detects an asserted CTS line after 30 seconds, the
terminal displays a CTS error message. This message remains
active for 2 seconds, then it is cleared from the display. After 30
seconds the terminal tests the CTS line again and if necessary
displays the CTS error message again. This cycle is repeated until
the communications problem is corrected and the CTS line is
non–asserted.
Momentary or Maintained Function Keys pseudo handshake
The MessageView terminal supports a pseudo handshake for
Momentary and Maintained Function Keys. The pseudo handshake
uses the Function Key Return Message Number tag defined in the
Terminal Setup Control Tags dialog. See Page 10–24.
If the Function Key Return Message Number tag is defined, the
MessageView terminal will return the message number associated
with the Function key each time the Function key is pressed. This
provides a means of signalling to the logic controller that a Function
key bit has been set or reset.
The Function Key Return Message Number tag is globally defined.
That is, the Return Message Number tag address is the same for all
messages using a Function key.