UNION SWITCH & SIGNAL GENISYS II System Hardware Installation
3-8 August 1999 SM-6900B Rev. 0.0
3.2.6 Isolation of Serial Port Signal Common
CAUTION
Application engineers should note that the serial commons for all GENISYS II serial ports
are connected directly to GENISYS II logic common and to each other. This means that
anything connected to any serial port signal common is also connected to the serial
common of all other connected devices. Furthermore, anything connected to the serial
common of any equipment that is directly connected to any GENISYS II serial port is
connected to GENISYS II logic common. This imposes serious restrictions on the
characteristics of the devices that can be directly connected to GENISYS II serial ports.
It should be noted, for example, that most commercial data modems connect their serial
common to earth ground either directly or through low-resistance. It should also be noted
that most data radios connect their serial common directly to their antenna ground. Both
of these conditions create a problem, since they introduce a connection between GENISYS
II logic common and earth ground. This type of connection has the potential to create
serious communication reliability problems by exposing GENISYS II units to ground loops
and to damage due to electrical surges.
This effectively means that devices like these must be connected to GENISYS II through a
serial line isolator, which provides a high-level of isolation between the signal commons of
GENISYS II and serial devices, such as modems and data radios.
Isolation between serial signal commons is also necessary when serially connecting GENISYS II
units to equipment powered by different batteries. As rail traffic control battery power supplies
are generally required to float with respect to ground, significant potential differences can
develop between the battery negatives. These potential differences can wind up being equalized
by the connection between serial commons. This situation poses a threat both to communication
circuit reliability and the electrical integrity of the connected GENISYS II units. In addition,
interconnection of battery commons by any means is not a recommended practice. This situation,
too, can be remedied by introducing a serial line isolator in the serial lines connected to
GENISYS II units. It is strongly recommended that all GENISYS II units NOT connected to the
same battery power supply be interconnected serially using communication devices that provide
serial common isolation. Furthermore, care must be exercised to insure that devices that ARE
serially connected directly to a GENISYS II unit DO NOT have serial connections to devices that
might ground serial common.
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