Solartron Mobrey 7828, Technical Manual

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Electrical Installation 7828 Density Transmitter Technical Manual
3-8
78285000_AB
3.4.3 RS485 TO RS232
Converters are available from a number of sources, and can range from simple, in-line
devices which simply plug into a PC’s RS232 port, to programmable devices with full
isolation between the two networks.
Note: 7828 uses a half-duplex implementation of RS485, such that the A and B signals
are used for data transmission in both directions. This requires that the RTS line is
toggled to indicate the transmission direction. This can be done by the host computer, or
automatically by an RS485/232 converter which has the facility to do so. If you are using
Windows NT on your PC, you should use a converter which automatically changes RTS,
(as detailed below), otherwise the link may not work correctly.
For simple installations, where the following conditions are valid, a simple in-line
converter will be satisfactory:-
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the Modbus network is less than about 50m (150ft)
•
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the number of devices on the bus is low
•
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no common mode problems
Solartron recommends the K485-ADE converter, manufactured by KK Systems Ltd and
obtainable from Solartron (part number 550003520); this will work with Windows 3.1, 95,
98, and NT. The converter’s RS485 connection is a 9-way plug, and the RS232
connector is a 25-way socket, which may require an adaptor to plug into the PC’s RS232
port (commonly a 9-way plug).
Solartron’s ADView software (part number 7828 1A) includes:
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ADView software
•
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K485-ADE RS485/RS232 converter
• =
9-way socket for connections to the 7828
•
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PC adapter (25-way plug / 9-way socket)
The K485-ADE converter derives its power from the PC’s RS232 port RTS or DTR line,
which must be held permanently in the high state. This is normally adequate for short
distances where there are only a few devices on the network. However, the ability of the
port to supply sufficient power is not guaranteed, especially for laptop PCs, and it may be
necessary to connect an external power supply. This may also be necessary if you are
using Windows NT. In this case a 9V supply can be connected between Pin 9 (+) and Pin
5 (GND) of the RS485 connector. See the manufacturer’s technical information for full
details.
Connections are shown below. Note that the RS485 signals are reversed between the
7828 and the converter: A goes to B and vice-versa.