15
3.5.2.2
Line Termination - End of Line Resistor
The MRS-485 and DVP-485B adapters are wired in the standard 2W-Modbus circuit definition with selectable built-in terminating resistors
at the ends of the RS-485 bus. The MRS-485 and DVP-485B adapters provide integral termination for the end of line resistors (EOL).
The terminations use 4-pin connector (for MRS-485) and 3-pin (for DVP-485B) with a jumper to select termination: The user selects no
termination or one of the two Modbus line termination options. The MRS-485 has two line termination options provided and DVP-485B
adapters have 120-ohm termination options provided onboard.
Figure 3-11 End of Line Resistor
Place the EOL jumper on one of the following positions:
3.5.2.3
Determining EOL Use
At the baud rate of 19200 (default baud rate for Macurco MRS-485 and DVP-485) and with cables less than 1,000 ft. in length, termination
resistors are not recommended.
At the baud rate of 19200 and with cables longer than 1,000 ft., termination resistors are recommended.
It is recommended to use an RS-485 type of cable and 120 ohms for termination resistor.
An RS-485 network requires a 3-wire cable: a twisted pair and a third wire. It is difficult to tell whether shielding is required or not in a
particular system until problems arise, so it is recommended to always use shielded cable.
When using termination resistors use only 2 resistors, one at each end of the RS-485 transmission line (i.e. 1 at the DVP-485B and 1 at
the last/farthest MRS-485).
3.5.2.4
Using Other Baud Rates
For any other baud rates calculate when (at what length) termination resistors are required. Calculate the length as 1000 ft. divided by
the ratio between the new baud rate and 19200.
For example, if new baud rate is 9600
1000 / (9600/19200) = 1000 / 0.5 = 2000 (ft.)
Use termination resistors when cables are longer than 2000 ft.
For example, if new baud rate is 38400
1000 / (38400/19200) = 1000 / 2 = 500 (ft.)
Use termination resistors when cables are longer than 500 ft.
Note: Other manufacturers may have different recommendations on wire length and EOL resistor usage.
