Manual 104i-COM-8SM
20
Appendix A: APPLICATION CONSIDERATIONS
INTRODUCTION
Working with RS-485 devices is not much different from working with standard RS-232 serial
devices and this standard overcomes deficiencies in the RS-232 standard. First, the cable
length between two RS-232 devices must be short; less than 50 feet. Second, many RS-232
errors are the result of noise induced on the cables. The RS-485 standard permits cable
lengths up to 4000 feet and, because it operates in differential mode, it is more immune to
induced noise.
A third deficiency of RS-232 is that more than two devices cannot share the same cable.
This is also true for RS422
but RS-485 offers all the benefits of RS422 plus allows up to 32
devices to share the same twisted pairs
. An exception to the foregoing is that multiple
RS422 devices can share a single cable if only one will talk and the others will always
receive.
BALANCED DIFFERENTIAL SIGNALS
The reason that RS422 and RS-485 devices can drive longer lines with more noise immunity
than RS-232 devices is that a balanced differential drive method is used. In a balanced
differential system, the voltage produced by the driver appears across a pair of wires. A
balanced line driver will produce a differential voltage from ±2 to ±6 volts across its output
terminals. A balanced line driver can also have an input “enable” signal that connects the
driver to its output terminals. If the “enable” signal is OFF, the driver is disconnected from
the transmission line. This disconnected or disabled condition is usually referred to as the
“tristate” condition and represents a high impedance. RS-485 drivers must have this control
capability. RS422 drivers may have this control but it is not always required.
A balanced differential line receiver senses the voltage state of the transmission line across
the two signal input lines. If the differential input voltage is greater than +200 mV, the
receiver will provide a specific logic state on its output. If the differential voltage input is less
than -200 mV, the receiver will provide the opposite logic state on its output. The maximum
operating voltage range is from +6V to -6V allowing for voltage attenuation that can occur on
long transmission cables.
A maximum common mode voltage rating of ±7V provides good noise immunity from
voltages induced on the twisted pair lines. The signal ground line connection is necessary in
order to keep the common mode voltage within that range. The circuit may operate without
the ground connection but may not be reliable.
