Siemens MJ-X Series, Installation Manual

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MJ-X Communications Module
4
RTU
Modem Modem
MJ-X
RS-232 Cable
50 ft Max.
RS-232 Cable
50 ft Max.
Figure 2-3 Modem Connections
2.2.2 RS-485
The maximum distance for RS-485 is 4000 feet, either from device to device or for an entire loop. The
recommended cable for RS-485 is shielded, twisted-pair, 22 AWG. This type of cable consists of jacketed wire that
is twisted together and wrapped in a shield to prevent electromagnetic interference from affecting the data signal. A
maximum of 32 devices can be connected to an RS-485 network. Typical network configurations are loop, star, and
open-ended. These are shown in Figure 2-4. The recommended network configuration is a loop. This will ensure
that all devices remain connected to the master unit in the event of a line break. If the loop configuration does not
allow you to place the equipment in the desired location, connect the network open ended.
If either the star or open-ended configurations are used, termination resistors must be installed at the end devices.
For more information on installing RS-485 networks, refer to Siemens publication SG-6028,
Installing the
ACCESS™ System . The following characteristics determine the maximum length and data rate of an RS-485
network.
Impedance : The maximum transfer of energy occurs when the cable’s terminating impedance matches the
characteristic impedance of the cable. If the cable’s characteristic impedance is 120 ohms, use a 120 ohm
terminating resistor.
Capacitance : Wire capacitance affects the rise and fall times of a transmitted signal and, as a result, limits the
signaling (baud) rate of the data. The higher the capacitance, the lower the maximum baud rate. Additionally,
higher capacitance requires higher output current to drive the line. Since capacitance is proportional to wire
length, it can limit the maximum cable length for a given baud rate.
Cable Size : Cable size, along with capacitance and DC resistance, determines whether the cable is suitable for
the application. A 22-gauge cable will provide a proper balance of capacitance and resistance.
DC Resistance : DC resistance determines the maximum length of wire allowed for an application. Determine
the maximum length from the resistance of each twisted pair along with the device termination resistance. The
use of surge protectors with series resistance also contributes to loop resistance.