Rs485 Electrical Configuration; Termination Resistors; Bias Function; Network - VSCOM ModGate Plus User Manual

3 Hardware Description
For RS232 the assignment is the same as on any PC (Com1/2). This is required by RS232.
Please note the GND signal in RS485 modes (and RS422). This signal must also be connected
between the serial devices. So in reality there is not a 2-wire connection. With the exception of
very special configurations, a serial cable without GND violates the specifications for RS485.
Also notice that the pins in the 4-wire modes are in symetric order in respect to its polarity having
the positive signals on 2 & 3 and the negative signals on 1 & 4 .
3.3. RS485 Electrical Configuration
In typical RS485 installations certain electric conditions have to be configured. Simply connecting
cables is not enough to fulfill the specifications of RS485. For ease of installations the ModGate
Plus Gateways provide these functions for often used parameters. They are activated by software.

3.3.1. Termination Resistors

The use of long communication lines in RS485 mode requires the installation of termination resis-
tors. These must match the impedance of the cable. Typical cables in Twisted-Pair configuration
have an impedance of about 120Ω. In RS485 the typical configuration requires one resistor at each
end of the cable. This resistor is activated via the webbrowser.

3.3.2. BIAS Function

RS485 requires a BIAS option for the communication lines. This will guarantee stable electrical
levels on the cables, even at times when no station is transmitting data. Without BIAS there will
be noise on the cable, and sometimes receivers can not detect the first characters of a beginning
communication.
The serial ports of the ModGate Plus Gateways do not require adding BIAS. This function can be
added to the cable for other hardware on the RS485 bus.

3.4. Network

The ModGate Plus connects to Ethernet, and with WLAN option it may use Wireless LAN or
Ethernet at customers choice. Both interfaces are enabled and configured, a ModGate Plus with
WLAN accepts connections in parallel.
3.4.1. WLAN Configuration
The pre-defined operation mode is as Access Point, providing an open wireless network. Any
computer with WLAN equipment may contact the ModGate Plus with WLAN. This operation
mode is implemented to assist modern Windows and other operating systems, where the Ad-hoc
Mode is removed.
However the AP-Mode is not encrypted by default. As one result any station can read the data
transferred to the ModGate Plus with WLAN. This also includes the passwords. Therefore the
recommended method is to use the Ethernet connector for the first configuration.
June 2018 ModGate Plus User Manual 14