3.11 RS422
EIA-422 or RS422 is a physical layer electrical specification of a four-wire, full-duplex, point to point or
multi-drop serial connection.
RS422 is a standard interface that is suitable for industrial applications. It was created for the
construction of multi-drop data busses, between a main computer and a number of terminals. The interface is
balanced, uses a four-wire format and is relatively immune to interference. The interface changes polarity of the
wire pair depending on whether a 1 or a 0 is to be transmitted.
RS422 was originally designed to handle 10 units and can now be set up with up to 32 units connected.
The maximum recommended distance is 1200m at 100kb/s (100 kilobits per second). RS422 can be integrated
with RS485, RS423-A and RS449 via converters.
The InSpec serial ports support RS422.
3.12 RS485
EIA-485 or RS485 is a physical layer electrical specification of a two-wire, half-duplex, multi-point serial
connection. It uses a differential form of signalling where the voltage difference between the wires coveys the
data. One polarity of voltage indicates a logic 1, the reverse polarity indicates logic 0. The InSpec serial
ports.support RS485.
3.13 Line Termination and Fail-Safe
It is recommended to terminate the line with a circuit of equal impedance to the characteristic impedance
of the transmission line. For RS422 and RS485 a 120-ohm resistance is recommended.
The purpose of the termination is to prevent the reflection of data at the ends of the cable. If there is no
active driver on the network the line can be forced into a known idle state with a fail-safe circuit. Without this
circuit it is likely that the line will pick up noise and falsely trigger the receivers leading to problematic
communications.
To Next Page
Loading...
Need help?
General
