Contemporary Control Systems BASgatewayLX - Manual (Page 10)

td100300-0Id
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note: If using shielded cable, connect to chassis at only one point. Wire size may
be dictated by electrical codes for the area where the equipment is being installed.
Consult local regulations.
Observe in Table 1 that 10BASE-T segments can successfully use Category 3, 4 or 5
cable — however, 100BASE-TX segments must use Category 5 cable. Category 5e
cable is highly recommended as the minimum for new installations.
The Ethernet port of the gateway employs Auto-MDIX technology so either straight-
through or crossover cables can be used to connect to the network.
ModBus port
This port uses an isolated EIA-485 transceiver connected to a 5-pin removable connector.
The transceiver + and – lines tie to + and – pins of the connector. The SC pin ties to the
transceiver ground. You MUST connect the SC pin to the Modbus segment signal
common — or EIA-485 communication will fail!
In Figure 9 a simplified schematic of the isolated EIA-485 transceiver is shown.
Notice that the transceiver common (SC) is electrically isolated from the gateway
power supply common (COM) through the use of opto-isolators and an isolated DC-DC
converter. An isolated EIA-485 transceiver offers less chance of damage to the
transceiver from high common-mode voltages, but there is still a risk. Surge
suppression is applied between the two differential inputs and between each input and
ground. Since the transceiver is electrically isolated, the transceiver circuit common
must be brought out for connection to the other devices on the EIA-485 network. This
is accomplished by pin SC. The differential pair is labelled + and – and requires a
three-wire connection to the EIA-485 network. Connecting other isolated EIA-485
devices is no problem since each device requires a 3-wire connection. Just make
corresponding connections. For non-isolated EIA-485 devices, usually only a 2-wire
connection is provided. The third wire is the power supply common. In this situation,
the SC connection MUST be made to the power supply common of the non-isolated
device. Refer to the device vendor instructions for connecting such a device.
In an application where one connection is made to Ethernet and the other to EIA-485,
the location of the gateway is probably at the end of the EIA-485 bus segment and
therefore both bias and termination must be applied to the segment end. The product
is shipped with bias and termination applied, but this can be changed by removing
three jumpers. This should be done if the gateway is to be connected anywhere
between the end EIA-485 devices.
The gateway can address up to 30 Modbus devices on the local bus at baud rates
from 2.4 to 115.2 kbps.