REV 0.0 April 18, 2023
Page 3
Shielding
Shielding is required for all Gateway-connected cables. See the Appendix for identification by ABM model number of
the sensors discussed in the following shielding rules:
•
Most important: the shield shall be connected to earth-ground in only one location. Connections at multiple
points will create a ground-loop that can result in significant ground differential voltages and currents, which
can adversely affect communication and even damage installed equipment.
•
Ideally, the sole earth-ground connection to the system’s cable shield should be made at either end (as
opposed to somewhere in the middle) of a multi-drop cable run. In most cases, especially if a line-powered
DC supply is used, it is best to ground it at the power supply.
•
The shields of all low-voltage cables should be connected together. Twisting together is acceptable, but
screw terminals, wire nuts or soldered connections are more reliable.
•
The installer must prevent the bare shield wires from touching any metal housing or any other conductors.
•
AC powered sensors must have an earth-ground wire directly connected to Terminal 6 for electrical safety as
part of the AC supply. Terminal 6 is internally connected to Terminal 3 on all sensors. Therefore, the cabling
system shield can use Terminal 3 on one AC sensor as its grounding point – as long as it is not already
grounded elsewhere.
•
DC powered sensors are not required to have an earth-ground wire connecting to Terminal 3 or 6, as low-
voltage installation rules do not require such. However, DC sensors with metal housings may be indirectly
grounded through their mounting – for instance, where a metal or otherwise conductive tank is grounded to
earth through its base and the sensor is installed without an isolating mount such as an aiming device. If a DC
sensor is not indirectly grounded in this manner, nor grounded through an earth-ground wire connected to
either Terminal 3 or 6, then the system shield wire should be connected to Terminal 3 or 6.
The above shielding rules are illustrated throughout the configuration examples in Section D.
RS485 Common Connection
While it is common practice to implement RS485 wiring with only two wires (“A” and “B”), this is risky over long
distances and multiple network nodes. Best practice, per the RS485 standards, is to provide a Common connection
between nodes.
The Gateway’s Common connection is the black wire of the red/black pair of both pigtail cables. The white/black pair
of the 2-pair cable provides the A/B connections.
For sensors that are not powered through the Gateway, such as AC powered sensors, the Common connection is to be
made through Terminal 4, which is Common for both the RS485 and the mA loop current output. For all DC powered
sensors, Terminal 4 connects internally to Terminal 7 (labelled “L2/N”).
RS485 Network Initialization
ABM sensors are supplied with a factory-default SID = 2, unless they were specifically ordered from ABM for a
Gateway network configuration with a pre-assigned Serial ID (SID) sequence. The connection of more than one sensor
having the same SID will result in communications failure.
Therefore, installation of more than one factory-default sensor to the Gateway requires that the installer only add one
sensor at a time to the RS485 bus (the black/white twisted pair) to re-assign its SID to a unique number, as described
in the Cellular Gateway User Manual.
