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Appendix B Serial Port Information
© National Instruments Corporation B-5 PCMCIA Serial for Windows 98/95
Termination
Because each differential pair of wires is a transmission line, you must
properly terminate the line to prevent reflections. A common method of
terminating a two-wire multidrop RS-485 network is to install terminating
resistors at each end of the multidrop network. If you daisy-chained
multiple instruments together, you need a terminating resistor at only the
first and last instruments. The terminating resistor should match the
characteristic impedance of the transmission line (typically 100–120
Ω
).
National Instruments offers an optional DB-9 RS-485 termination
connector that contains embedded terminating resistors for easy
termination.
Figure B-3 shows a multidrop network using terminating resistors.
Figure B-3. Multidrop Network Using Terminating Resistors
DTE vs. DCE
Data Terminal Equipment (DTE) and Data Communications Equipment
(DCE) were the terms used in the RS-232 specification for the types of
equipment on either end of a serial connection. (A DCE is called Data
Circuit-Terminating Equipment in Revision D of the RS-232 specification.)
In general, DTE and DCE refer to computer equipment and modems,
respectively. Because the RS-232 specification mainly involves connecting
a DTE directly to a DCE and vice versa, the pinouts are defined so that
cabling is simple. That is, a cable connected a computer to a modem by
wiring pin 1 to pin 1, pin 2 to pin 2, and so on. This method is commonly
known as
straight-through cabling.
MASTER
Slave 1
100
Ω
Slave 2 Slave n
100
Ω
Tx
Tx
Rx
Tx
Rx
Tx
Rx
Rx