Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

2.7.6 Synchronous communications
2.7.7 RS-485
Fundamentals of SCADA communications 41
The major difference between asynchronous and synchronous communications with
modems is the need for timing signals.
A synchronous modem outputs a square wave on pin 15 of the RS-232 DB-25
connector. This pin 15 is called the transmit clock pin or more formally the DCE transmitter
signal element timing pin. This square wave is set to the frequency of the modem’s
bit rate. The attached personal computer (the DTE) then synchronizes its transmission of
data from pin 2 to the modem.
There are two interchange circuits that can be employed to change the operation of the
attached communications device.
These are:
Signal quality detector (CG, pin 21)
If there is high probability of error in the received data to the modem because of poor
signal quality this line is set to OFF.
Data signal rate selector (CH/CI, pin 23)
If the signal quality detector pin indicates that the quality of the signal is unacceptable
(i.e. it is set to OFF), the terminal may set the pin 23 to ON to select a higher data rate;
or OFF to select a lower data rate. This is called the CH circuit.
If, however, the modem selects the data rate and advises the terminal on pin 23 (ON or
OFF), the circuit is known as circuit CI.
Disadvantages of the RS-232 standard
System designers have tended to look for alternative approaches (such as the RS-422 and
RS-485 standards) because of the following limitations of RS-232:
• The restriction of point-to-point communications is a drawback when many
devices have to be multidropped together
• The distance limitation (typically 50 meters) is a limitation when distances
of 1000 m are needed
• The 20 kbps baud rate is too slow for many applications
• The voltages of –3 to –25 volts and +3 to +25 volts are not compatible with
many modern power supplies (in computers) of +5 and +12 volt
• The standard is an example of an unbalanced standard with high noise
susceptibility
RS-485 permits multidrop network connection on two wires and provides for reliable
serial data communication for:
• Distances of up to 1200 m
• Data rates of up to 10 Mbps
Up to 32 line drivers permitted on the same line and up to 32 line receivers are permitted
on the same line.