Altronic CDM-2100 Operating Manual - Page 10

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9.0 RS-485 COMMUNICATIONS

The CDM-2100 gauge is part of a system that has been carefully designed to

easily interface to popular computers, terminals, programmable controllers and

Altronic instruments. The gauge communicates in the Modbus RTU protocol.

9.1

MASTER/SLAVE OPERATION:

The gauge's RS-485 communication system is designed as a master/slave

system; that is, each unit responds to its own unique address (node number)

only after it is interrogated by the master (computer). One master and up to 32

slaves can communicate in the system. The units communicate with the master

via a polling system. The master sends a command and only the polled slave

responds. The slave modules can never initiate a communications sequence. A

simple command/response protocol must be strictly observed.

9.2 NODE NUMBER:

The node number is used in the system to identify the desired slave unit being

polled. The node number can be any numeric value from 1 to 99 although

only 32 devices can be served on a single communications port. This number

range (1 to 99) is allowed so that if device grouping by function or application

is desired, it can be implemented using the first digit as the group or engine

number and the second as the unit number. For example, 53 could be used to

identify the number 3 slave unit mounted on engine number 5.

9.3 BAUD RATE:

Baud rates available are 9600, 19200, 38400, 57600, 115200.

9.4 HALF-DUPLEX OPERATION:

The RS-485 system employed uses two wires for communication and cannot

send and receive data at the same time over the same two wires making it a

half-duplex system. When the master is in the transmit mode, the slave is in the

receive mode and vice-versa.

9.5 ELECTRICAL OPERATING RANGE:

RS-485 is a communications standard to satisfy the need for multi-dropped

systems that can operate at high speeds over long distances. RS-485 uses a

balanced differential pair of wires switching from 0 to 5 volts to communicate

data. RS-485 drivers can handle common mode voltages from -7 to +12

volts without loss of data, making them an excellent choice for industrial

environments.

9.6 COMMUNICATIONS PARAMETERS:

The following must be set by the master to communicate with the slaves:

Baud Rate:

9600 (DEFAULT) others available, see section 10.3



Data Bits:



Stop Bits:



Parity:

None



9.7

COMMUNICATIONS WIRING:

The RS-485 wiring diagram illustrates the wiring required for multiple slave

unit hookup. Note that every slave unit has a direct connection to the master.

This allows any one slave unit to be removed from service without affecting

the operation of the other units. Every unit must be programmed with a unique

address or node number, but the addition of new units or nodes can be in

any order. To minimize unwanted reflections on the transmission line, the bus

should be arranged as a trunk line going from one module to the next. Random

structures of the transmission line should be avoided. Special care must be

taken with long busses (500 feet or more) to ensure error-free operation. Long

busses must be terminated with a 120 ohm resistor between the terminals

marked RS-485 A and RS-485 B at the master only. The use of twisted pair

shielded cable will enhance signal fidelity and is recommended. To prevent

ground loops, the shield should be connected to the shield terminal at the

master only.

CDM-2100 OM 3-21