Appendix C
Serial Port Information
PCI Serial for Windows 95
C-6
©
National Instruments Corporation
Figure C-4 shows a transmission line using bias resistors.
Figure C-4. Transmission Line Using Bias Resistors
Rather than using two 620
Ω
resistors at one node, you can also increase
the value of the resistors and put them at every node. For instance, if there
are eight nodes in a system, you can use 4.7 k
Ω
resistors at each node to
effectively achieve the same result.
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.
620
Ω
Bias Resistor
+5
A
Tx
B
620
Ω
Bias Resistor
100
Ω
Termination Resistor
