Reference Manual
00809-0100-4738, Rev DA
May 2006
Rosemount 3095FB
2-12
NOTE
The RTU may already provide one RS-485 bus termination.
Pull-down (B) and Pull-up (A)
These switches are used to put the RS-485 bus into the idle state. Set these
switches either to both ON (idle state), or to both OFF (lets the bus float). If a
transmitter has multiple communication errors, set these switches to ON (idle
state).
NOTE
Only one device on an RS-485 bus should set the bus to the idle state. In
some installations, the RTU might maintain the bus in the idle state when the
RS-485 bus is inactive.
Baud Rate (S1 and S2)
Table 2-2 identifies four available baud rates for RS-485 Modbus
communications.
Table 2-2. Baud Rate Settings
Configuring RS485 and Security Jumpers
When shipped from the factory, the RS-485 switches are all set to off, and the
baud rate switches are set to 9600 (see Table 2-2).
To reposition the jumpers, follow the procedure described below.
1.
If the transmitter is installed, remove power.
2.
Remove the housing cover opposite the field terminal side. Do not
remove the transmitter covers in explosive atmospheres when the
circuit is live.
3.
Remove the LCD meter if present.
4.
Locate the switches on the output electronics board (see Figure 2-5),
and then move the switch to the desired setting.
5.
Reattach the transmitter cover. Transmitter covers must be fully
engaged to meet Explosion-Proof requirements.
S1
S2
Baud Rate
OFF
OFF
1200
ON
OFF
2400
OFF
ON
4800
ON
ON
9600
