Mercury 2+ Operator Terminal User Manual
Page 39 of 82
2019r0
In multidrop mode the master device polls the slave Mercury 2+
Terminals. Up to 15 Terminal systems can transmit to a single host
computer via a single twisted pair cable utilising RS485 levels
(two pairs if RS422 is used). The Multidrop master initiates all
communications and the slaves can only reply when requested.
Multidrop operation is supported over the RS485 and tristate
controlled RS422 communications link between I.S Interface
Modules. So these must be installed even if operation is in non
hazardous areas.
The Mercury 2+ LCD screen in multidrop mode operates in the
standard page format, with the exception that the eighth line is
used as the editing area for composing blocks. To avoid deletion
of data being composed on Line 8, host operators should use lines
1 to 7.
In the case of the host computer transmitting a Clear Screen escape
sequence, in Multidrop mode, lines 1 to 7 of the Screen are
cleared. Line 8 is not affected.
On Line 8, an alphanumeric key press causes the character to
appear on the bottom line of the screen at column 6. Up to 30
characters can be composed into a message. To edit use the
backspace key, BS (F8 in Numeric mode). When complete, press
ENTER to queue the message for transmission.
If a function key is activated while a message is being composed,
the function code is added to the block buffer ahead of the
message, without affecting the composition of the message.
Messages are blockbased and therefore indivisible. So the
software handshaking facility Xon/Xoff is automatically disabled
if Multidrop Operation is selected in the Setup menu (see Table
1).
The Mercury terminal does not inhibit received characters being
displayed on the 8th line of the screen. So a message being
composed may be overwritten on the screen, although it will still
be composed correctly into a block.
Multi-Drop
Telemetry
Multi-Drop Mode
Screen
