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User manual
25.7.2007
Hardware V1.1
DCS771B
USB-to-RS-485 converter
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INTRODUCTION
DCS771B is used to add an RS-485 connection in a PC computer. It is connected in a USB port, and the
drivers create a virtual COM port that can be used just as an ordinary COM port. Even the baud rates and bit
configuration can be changed in Windows port settings – these require no jumper settings. DCS771B can be
used with a variety of protocols, including Nokeval SCL and Modbus ASCII and RTU.
DCS771B can give a small amount of power from the USB to the RS-485 devices. For example, two Nokeval
radio receivers RTR970's can be powered. If more power is needed, it can be fed to the bus conveniently
with a dedicated screw terminal.
To aid troubleshooting, DCS771B is equipped with three indicator LEDs. The 485 bus is connected with
screw terminals eliminating the need to make solder joints on the cables.
Due to individual serial number on every device, these devices retain the COM port number if detached and
replugged, even in a different USB port. This increases reliability.
The difference between DCS771B and the previous DCS771 is that the B model can make an automatic
decision if the supply voltage to J2 is taken from USB or an external source. The jumpers are also different.
SPECIFICATIONS
USB connection
Connection: USB 1.1 or 2.0
Bus chip: FTDI FT232BM
Consumption: 100 mA
Connector: USB-B (USB-A-B cable
included).
RS-485 bus
Baud rates Any up to 115200 bit/s
Data bits 5..8
Parity All modes supported
Stop bits 1, 1.5, 2
Bus length 1000 m
Devices on bus max 32 normal load devices
or 128 1/4 load devices
Load 1/4 load
Power from USB
Voltages 10 V 120 mA or 24 V 50 mA
Short circuit protect Yes
General
Dimensions: 66x67x28 mm
Operating temp: 0...60 °C
Galvanic isolation: No
Regulations
EMC immunity
EN 61326
EMC emissions
EN 61326 class B
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DRIVERS
Installing
The USB interface chip needs two drivers: one for USB bus and the other to create a virtual serial port.
The drivers can be obtained from a Nokeval Software CD or downloaded from the USB chip manufacturer:
www.ftdichip.com (Drivers, FT232RL, VCP). The installation below assumes using CD, but using
downloaded drivers is quite similar.
Insert the Nokeval Software CD and plug in the device. Windows should detect it and start installing
automatically:
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When USB bus driver ftdibus.inf is installed, Windows wants to install the virtual serial port driver, that makes
the device to look like an ordinary COM port.
Install in the same way as the bus driver. When all is complete, the OK indicator on the device should be lit.
Finally it is necessary to find out, which COM port represents the device. Open Control Panel, System,
Hardware, Device Manager. In the device tree, expand Ports, and there should be USB Serial Port (COM3),
for an example.
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CONNECTING
485 bus connections
DCS771B has four terminals for the RS-485 bus: Power supply +, Common, D1, and D0. These are
connected one-to-one to the other devices on the bus. The D1 and D0 lines carry the data in both directions
(one direction at a time, i.e. Half-duplex). Common wire is needed to equalize the ground potential of the bus
devices. It must be connected even if no power is taken from the DCS771B.
However the older Nokeval devices do not have a Common terminal available – in that case connect only D1
and D0.
If the bus devices are to be powered via the DCS771B, the power is provided in the bus connector terminals
1+ and 2-. The DCS771B can obtain that power with various ways, see the next section.
There has been quite rich use of names for the signal lines. Even EIA/TIA recommended ”A” and ”B” have
been used with two meanings. Nokeval has decided to use the names defined by Modbus standard, D1 and
D0. The table below shows typical names for the lines:
D1 D0
+ -
B A
A B
The cable should be of a twisted pair, minimum diameter 0.5 mm (0.2 mm2, 24 AWG). A shielded cable may
be used to increase immunity to disturbances. The shield should be grounded at one end only.
321
Rx
Tx
OK
RS-485 device
C
D1
D0
4
+
321
Power
supply
(optional)
+
-
Mascot
(optional)
USB
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Power options
DCS771B can provide the supply voltage
for the bus devices. It can obtain this
power with three ways:
From the USB
Jumper JP1 is set in position ”Auto” and
the voltage selected with JP2 (options are
10 or 24 V). The maximum current is
stated in the specifications.
External power supply
The external supply is connected in
connector J1. Jumper JP1 is
recommended to be set in position
”External”. The voltage and current depend
on the external power supply – the
DCS771B only routes it from a connector
J1 to J2.
External Mascot
JP1 is recommended to be set in
”External”. DCS771B routes the Mascot
voltage from connector J3 to connector J2.
When J5 is in the Auto position, DCS771B detects if there is an external power supply connected in J1 or J3,
and if not, then generates the supply voltage to J2 from the USB. In External position, DCS771B never uses
USB power.
To open the case and access the jumpers, unscrew the two screws in the bottom.
Terminating and fail-safing
If the bus is longer than some dozens of meters or if the highest baud rates are used, the bus must be
terminated in order to avoid reflecting the signal from the bus ends. The reflections will cause bit errors.
The bus topology should be daisy-chain, although short stubs (a couple of meters) are acceptable. The first
and the last device on the bus should be terminated. Most RS-485 devices provide some means of
terminating without external components, typically closing a jumper or a DIP switch.
Fail-safe resistors are needed to ensure the correct state on the bus when nobody is transmitting. They
make at least 0.2 V voltage between lines D1 and D0. Modbus specification recommends that the bus has
only one device fail-safing. So it is natural to have the fail-safe on the master of the bus.
DCS771B can do both terminating and fail-safing with jumper settings. Their default setting is on. They may
be changed by opening the case – unscrew the two screws in the bottom.
The termination provided by DCS771B is an AC termination (there is a resistor 110 ohms and a capacitor 1
nF in series). It is enabled by closing the middle jumper on J7. It should be enabled, if DCS771B is the first or
the last device on the bus.
The fail-safe resistors are 500 ohms to 5 V, as recommended in Modbus specifications. They are enabled
by closing the first and the third jumper on J7. They should be closed, if there is no other fail-safing devices
on the bus.
J5
Auto*
External
J7
None
*FS+Term
+
Common
D1
D0
Common
Free
+
Mascot
USB
J2 J1
J3
J9
J7
JP2
J5
10 V*
24 V
JP2
* = factory set
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TROUBLESHOOTING
The indicator LEDs in the DCS771B are very helpful when something is wrong with the communications.
Step 1: Drivers
Check if the OK LED is lit in the DCS771B. If not, reinstall the drivers.
Step 2: Transmission
Start your PC application and check that the Tx LED is blinking on the DCS771B. If not, there is something
wrong with your application – a wrong COM port probably.
Step 3: Reception
If the Tx LED is blinking but Rx is not, then your slave device is not responding. Check that it is powered up,
it is wired correctly, and that the serial settings are correct.
Step 4: Bus voltage
Use a multimeter to measure the voltage between D1 and D0 lines. It should be at least 0.2 V, D1 being
more positive. If it is 0 V, there is a short-circuit, or wire break, or no fail-safe enabled on any device.
Measure the voltage at every device, ensuring the polarity is still correct (D1 more positive).
Yrittäjäkatu 12
37100 Nokia
Finland
Tel +358 3 3424800
Fax +358 3 3422066
www.nokeval.com
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