Dwyer DPM - Manual (Page 41)

Particulate Monitoring Systems
Installation & Operating Manual
Document No. 210-1003-M Page 34 ©2007
12.1 Modbus®/RTU RS-485 Networking Protocol
The RS-485 networking feature allows up to 32 control units to be connected to a multi-drop
communications network. When connected to the network, any device on the network may be
monitored and controlled by a remote device such as a PLC or Computer using Modbus®/RTU
protocol.
Network Hardware Description
RS-485 is a standard industrial network used for serial communications between multiple devices
from a single connection. Electrical communication signals are transmitted differentially providing
immunity to electrical noise and power supply variances. Signals are transmitted between devices
over a single twisted pair wire with shield. Communications are half-duplex (cannot transmit and
receive at the same time). Serial baud rate is fixed at 19,200 bps. The serial frame is fixed at 8 data
bits, 1 stop bit and no parity. Communications are supported over a maximum network length of 4000
feet. When connecting more than two devices on the network, all devices should be wired ‘in-line’
and not in a ‘star’ configuration.
A terminating resistor must be present at each end of the RS-485 network to eliminate transmission
reflections on the serial line. Some control units contain a two-position jumper to allow the device to
be either terminated (T) or un-terminated (U). The terminating resistor connected in the (T) position is
120Ohms. All other devices on the network that are not at a network end must be set to un-terminated
(U). For control units that do not contain a terminating jumper, a 120ohm ½ watt resistor may be
placed across the RS-485 +/- output terminals.
PLC Connection
Connection to plant PLC’s is dependant upon the communications ports available on the specific PLC
being used. Ensure that the communications port connected to is not limited to a PLC manufacturer’s
proprietary network protocol such as Allen-Bradley Data-Highway or Remote I/O. Modicon’s
Modbus® “Plus” protocol is also not supported. This is a Modbus® RTU protocol.
Some configuration may be required in the PLC to set the communications port to Modbus®/RTU.
RS-485 Communications Ports
A network connection may be directly wired to any communication ports that support half-duplex RS-
485 and Modbus®/RTU protocols. Refer to the PLC manufacturer’s literature for specific details on
connections to the PLC communications port.
Network connections to RS-232/RS-232C or RS-422 ports are supported with the addition of a
converter module. As with the RS-485 port, the Modbus®/RTU protocol must be supported for proper
operation. Converter modules are available from the factory. The converter module modifies the
voltage levels and wiring connections to allow different RS connections to work together. Converter
modules generally require their own power source, which must be provided in the PLC cabinet.
When using an RS-485 to RS-232 converter the converter must be setup so it will transmit when the
TD line is asserted.