Siemens MJ-X, Installation Manual

Page: 7 / 34

MJ-X
L
Communications Module
3
The distance between devices may be extended with modems, Figure 2-3. Modem connections are either dial-up,
short-haul, or radio-based. Dial-up modems are typically deployed when there is easy access to the telephone
connections, while short-haul modems are useful within a facility where there is access to the wiring. Consider
using a radio modem when laying cable for a land-line modem is impractical.
RTU MJ-X MJ-X
50 Feet
RS-232 Cable
50 ft Max.
Figure 2-2 RS-232 Connection
RTU
Modem Modem
MJ-X
RS-232 Cable
50 ft Max. RS-232 Cable
50 ft Max.
Figure 2-3 Modem Connection
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 Figures 2-4 to 2-6. 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 Section 7.2 RS-485 Twisted-pair Cable. 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.