Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

36 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
The RS-232 standard consists of 3 major parts, which define:
• The electrical signal characteristics such as the voltage levels and grounding
characteristics of the interchange signals and associated circuitry
• The interface mechanical characteristics of the mechanical interface between
DTE and DCE, and
• The functional description of the interchange circuits, such as the function
of the data, timing and control signals used at the interface between DTE
and DCE
2.7.2 Electrical signal characteristics
The RS-232 interface standard is designed for the connection of two devices called:
• DTE
Data terminal equipment (e.g. a computer or printer). A DTE device communicates
with a DCE device and transmits data on pin 2 and receives data on
pin 3 on a 25-pin D-type connector
• DCE
Data communications equipment, now also called data circuit-terminating
equipment in RS-232D/E (e.g. a computer or Modem). A DCE device transmits
data between the DTE and a physical data communications link
(e.g. telephone system). It transmits data on pin 3 and receives data on pin 2
on a 25-pin D-type connector
Figure 2.22
The connections between the DTE and DCE