Information Technology Reference
In-Depth Information
•
An active RTS line.
•
Bit changes on the transmit data line.
•
Sends of the X-ON/X-OFF flow control characters.
Slaves cannot send data unless they are selected.
In a four-wire operation RS-485 devices connect with two twisted pair cables with char-
acteristic impedance of 120
Ω
and general shielding. Each link requires a terminating load on
the ends of the cable.
14.3 Line drivers
Transmission lines have effects on digital pulses in the following ways:
•
Attenuation - The transmission line contains series resistance that causes a reduction in
the pulse amplitude.
•
Pulse distortion - The transmission line insulation produces a shunt capacitance on the
signal path and a series resistance and inductance of the conductors. This causes the
transmission line to distort the shape of the pulse. The two main effects are the block of
high frequencies in the pulse and phase distortion.
•
Noise - Noise is any unwanted electrical signals added to a signal. A digital system is
less prone to noise as it has only two levels and it takes a relatively large change in volt-
age to cause an error.
Table 14.1 shows the electrical characteristics of the different serial communication stan-
dards. The two main standards agencies are the EIA and the ITU.
Balanced lines use two lines for each signal line, whereas unbalanced lines use one wire
for each signal and a common return circuit (see Figure 14.3). RS-422 is a balanced interface
and uses two conductors to carry the signal (see Figure 14.4). The electrical currents in each
of the conductors are 180
out-of-phase with each other. Balanced lines are generally less
prone to noise as any noise induced into the conductors will be of equal magnitude. At the
receiver the noise will tend to cancel out.
The voltage levels for RS-232 range from ±3 to ±25 V, whereas, for RS422/ RS423 the
voltage ranges are ±0.2 to ±6 V. For very high bit rates the cable is normally terminated with
the characteristic impedance of the line; for example, a 50
°
Ω
cable is terminated with a 50
Ω
termination.
RS-422 interface circuits can have up to 10 receivers. They have no ground connection
and are thus useful in isolating two nodes. For two-way communications four connections
are required, the
TX+
and
TX-
on one node connects to the
RX+
and
RX-
on the other.
Nodes may have a direct RS-422 connection or can be fitted with a special interface
adaptor to convert from RS-232 to RS-422 (although the maximum data rate is likely to be
limited to the maximum RS-232 rate).
It should also be noted that the maximum connection distance relates to the maximum
data rate. If a lower data rate is used then the maximum distance can be increased. For exam-
ple, in some situations with a good quality cable and in a low noise environment, it is possi-
ble to have cable runs of 1 km using RS-232 at 1200 bps.
