55xGenComm1_29
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RS485 description
Again the RS485 communication standard defines only the connection between the equipment, not
the communication protocol which is described below. RS485 is a two-wire interface, capable of
allowing up to 32 devices to communicate with each other. This feature is called ‘multi-drop’. It
allows, for instance, a single Personal Computer equipped with RS485 interface to communicate
with between 1 and 31 generating set controllers on the same site, with only one cable between
them. The cable is connected from one device to another in ‘daisy-chain fashion’.
CAUTION!:- . RS485 cabling must be 120Ω cable, specified for use with RS485. 120Ω terminating
resistors must be fitted to the first and last devices on the bus. Some PC RS485 cards are already fitted with
this resistor, and in this case should not be fitted externally. If in doubt, consult the supplier of your PC RS485
card. If a 55x/52xx unit is the ‘last’ device on the bus, the internal terminating resistor MUST be enabled.
Recommended cable BELDEN 9841 120Ω RS485 cable.
NOTE:- . For more detailed RS485 connection details of the DSE controllers, including recommended
modem type, please consult the Link500Plus manual (55x modules) or 52xx configuration software manual
(52xx series modules).
Communication protocol description
Once all the pieces of equipment are connected together (RS232, RS485 or other), they must both
speak in the same electronic ‘language’ to enable each piece of equipment to understand each
other. These ‘languages’ are called protocols.
A simple protocol is ASCII (American Standard Code for Information Interchange) which is simply
the transmission of data as single characters using a simple code (i.e. letter A = 65, letter B = 66, a
space = 32).
More complex protocols may include some form of error checking to test the data to ensure it has
not been corrupted during the ‘conversation’.
The DSE 55x/52xx modules utilise the Modbus™ protocol for its communication. Modbus is a very
robust and secure master/slave protocol, widely used and supported by many pieces of equipment
and PC software. The module itself performs a ‘slave’ operation, maintaining silence on the
communication link until being sent instructions by the ‘master’ (normally a PC running P810 or
Link500Plus). The master can either request information from the module, or instruct the module to
perform a certain task, such as starting or stopping the engine. In the case of multiple module
(RS485) installations, the ‘master’ must include in its request, the identity of the module (SlaveID) it
wishes to communicate with. Each module must have a unique SlaveID so that only one module
will interpret the request and perform the task asked of it by the ‘master’.
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