ERE S n.c. DC485USB: Instruction Manual Revision: 1.0
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available choice, select the search into the computer, specifying the directory in which are stored the requested
files, i.e., the C:\FTDI Driver directory.
-At the end of this procedure the new hardware is installed as a new COM Port with an automatically assigned
new number, for example the COM4.
To verify the presence of the new COM port, and its number, enter in the Control Panel > System > Hardware >
Device Manager > Ports and verify the presence of the just installed new COM port. To verify the correct driver
association, enter in its Property and, selecting the Driver label, verify the use of the FTDI driver. If this driver is used
the DC485USB converter is correctly installed in the computer and may be used. The serial port characteristics may be
set in Property > Port Settings page. The maximum verified port speed is 115.200 bps.
3.0.2 Converter parameters.
In the final test each converter is loaded with some configuration parameters that identify the device every time it is
connected to a PC. All our converters are characterised with a unique identification code to avoid the creation of more
than one Virtual Com Port, one for each device, if you have more than one converter.
4.0 Signals of the RS – 485 Serial Port.
4.0.1 A and B lines.
The RS – 485 Port connector is characterised by two data lines, Aand B, and the Common Reference line (GND)
normally connected to the negative pole of the supply and to the earth. If a shielded cable is used, this point may be
used also to connect its shield.
The two lines are balanced respect to the common reference line (GND), that is, generally, the negative pole of the
supply voltage. It’s recommended to connect also the common reference line, between the converter and the device,
specially if they are quite far and if the cable may be subjected to the influence of a strong magnetic field or a
electrostatic discharge. In these cases is recommended the use of a shielded cable, with the precaution to connect its
shield only on one side, to obtain an equipotential shield. Both the Aand Blines are protected against overvoltages
by the use o transient voltage suppressors.
4.0.2 Power (PWR) line.
The DC485USB converter is equipped with an auxiliary output signal called Power (PWR) that may be used as a
remote switch of the external device. This is an active High signal and it’s referred to Ground (GND). The available
voltages are 5 Vdc, when is High and 0 Vdc if it’s Low. This output is driven by the DTR criteria of the Virtual COM
Port and, generally, is activated by the application software, that utilises the COM Port. If the application software does
not manage the DTR of the COM Port, this signal remains Low and the external device is turned OFF. To use
correctly this signal as remote switching ON/OFF is necessary to foresee a delay time between his activation and the
start of data flow, to allow the exhaustion of the turn ON transient of the controlled device. At the same manner, at the
end of the data flow, a delay time must be inserted between the data end and the turning OFF, to allow the complete
data acquisition. These delay times depends on the characteristics of the controlled device and, generally, a delay of 50
or 100 milliseconds is adequate and may be reduced, but must be experimentally verified.
For example, in the devices of the Radiant series, this signal is controlled by the configuration software, or by some
utility software such as the Radiomodem Test, in which the required delay times are internally set. This line, as the A
and B of the RS-485 serial port, is protected against overvoltages and is characterised by an internal impedance of
450 Ωtherefore it can deliver a current of 2 mA @ 4 Vdc output, generally adequate to drive a TTL digital input. It’s
possible to connect directly an optocoupler between this line and GND, without external resistors.
The following Figure 1 shows the correct timing diagram for a correct use of this signal.
Fig. 1: Timing diagram of the PWR line.
