TOSHIBA HC422B

Description

TIA/EIA-422-B (RS-422) is an industry standard specifying the electrical characteristics of a balanced
interface circuit. Other prefixes are commonly used with the RS-422 standard. These include EIA, EIA/TIA,
and RS; although previously correct, today TIA/EIA is the correct prefix. However, for simplicity, RS-422
wil be used throughout the rest of this application report. Also, a suffix letter denotes the different revisions
of the standard but various prefixes with the same suffix reference the same identical standard.
RS-422 was introduced to solve the limitation problems of single-ended standards like TIA/EIA-232-E.
Single-ended interfaces lack common-mode noise rejection capability; ideal for noisy environments. Also,
data rates are usually limited to less than 0.5 Mbps. A RS-422 interface may be implemented to overcome
these limitations.
A RS-422 driver can drive up to ten unit loads (4 kΩ to circuit common is one unit load). The driver is
capable of transmitting data across 4000 feet (recommended limit) of cable; but not at maximum data
rates (see Figure 3 ). Standard RS-422 drivers are assured to source and sink a minimum 20 mA across a
100 Ω load. This corresponds to a minimum differential output voltage, VOD, of 2 V across the load (see
Figure 1).
Figure 1. Terminated Configuration
The complement RS-422 receiver must be equal to or less than one unit load. This is represented by the
slope of the shaded region in Figure 2. The operating range of the receiver is defined between ±10 V and
is represented by the shaded area in Figure 2.
Figure 2. Receiver Operating Range
Also, RS-422 receivers have a ±200 mV threshold over the entire common mode range of ±7 V. A
differential noise margin ≥1.8 V is assured between the driver’s differential output swing and the receiver’s
threshold.
RS-422 drivers and receivers are designed for point-to-point and multi-drop configurations but not multipoint. For multi-drop configurations, a daisy chain is the recommended interface configuration.
Caution, at long distances or high data rates, termination is recommended to reduce reflections caused by
a mismatch in the impedance of the cable and the impedance of the receiver’s input. For further
information, see Section 3.
Significantly, the RS-485 differential interface standard is very similar to RS-422. However, there are
differences that distinguish the two standards from one another; which include the output stage of the
driver, the common mode range of the interface, the input resistance of the receiver, and the drive
capability of the driver. For more details concerning the comparison of RS-422 and RS-485, see AN-759
Comparing EIA-485 and EIA-422-A Line Drivers and Receivers in Multipoint Applications (SNLA023).
2 Cable Length and Data Rate
Cable length and data rate have an inverse affect on each other. When operating at either the
recommended maximum cable length or data rate, the other can not be obtained. For instance, it is not
possible to operate at 4000 feet when operating at 10 Mb/s or vice-versa.