Specifications
43
Schneider Electric
20 mA CL – RS232 – RS422 –
RS485 links (continued)
The RS232 link
The physical DB25-DB25 connection
Definition: DTE (Data Terminal Equipment) represents the terminal or computer.
Definition: DCE (Data Communication Equipment) represents the modem (or
printer).
b Link two devices of different types (DTE-DCE) with a direct PIN-to-PIN cable.
b Link two devices of similar type with a special cable:
v DTE-DTE, use a nul-modem cable,
v DCE-DCE, use a nul-terminal cable.
On computers the serial port is usually a DB9 or DB25 male. The parallel port is a
DB25 female.
Note: The terms "DCE" and "DTE" are not related to the type or gender of the
connector.
Standard RS-232-C does not clearly define whether the connector is male or female.
Its purpose is rather to standardise the function and utility of connector pins and the
voltage applied.
Definition: Parallel communication is the simultaneous transmission of 8 binary
elements (a byte). The 8 bits in the byte are sent onto the communications media
simultaneously.
Printers are often connected to computers with a parallel cable with DB25
connectors.
The maximum length of a parallel cable is 30 meters. A parallel cable that is too long
could falsify the transmitted data because of electrical noise and the deformation of
squared waves (bits at 1 and 0).
Definition: Serial communication is the transmission of bits one after another.
This method is used when the distance between the two machines is too great to link
them with a parallel connection. Note that computers and modems are connected
via a serial connection. Manufacturers usually comply with connection standard RS-
232-C.
The maximum recommended length for a serial cable is 30 meters, but longer cable
can be used if certain electrical characteristics specified in standard RS-232-C are
respected.
General characteristics:
b Electrical levels ± 15 V.
b Control lines: possible (RTS, CTS, DCD,
etc.) but not systematic depending on the
software and/or interface.
b Charge and polarisation of the line: no.
b Minimum connection (in DB9):
v pin 2 (Transmitted Data),
v pin 3 (Received Data),
v pin 7 (Signal Ground).
E52326
DB25 Female RS-232-C
DB25 to DB25 serial cable
DB25 Male RS-232-C
Computer
Interface
DTE
DCE
13
1
25
14
1
13
14
25
Guide to Integration
Modbus / Jbus