EIA-422

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EIA-422 , also standardized as RS-422 and worldwide also as ITU-T V.11 , is an interface standard for line-based differential serial data transmission . EIA-422 is specified in "Electrical Characteristics of Balanced Voltage Digital Interface Circuits (ANSI / TIA / EIA-422-B-1994) (R2000) (R2005)" .

description

EIA-422 only specifies the electrical properties of the interface, it does not define any protocol or pin assignment. It is envisaged that the EIA-422 specification will be referenced by other specifications. One example is VDCP . In case of doubt, when coupling different EIA-422 devices, their documentation must be observed.

In contrast to the asymmetrical serial interface according to the EIA-232 standard , EIA-422 requires symmetrical signal transmission , in which the transmitter drives both conductors of the signal line with voltages symmetrical around zero against signal ground, both conductors as twisted pairs with the same reference to the common shield and the receiver has a differential signal input. This reduces the influence of common-mode interference , which enables smaller signal amplitudes and higher data rates with a line terminating resistor .

The transmission from a transmitter ( transmitter , TX ) to a receiver ( receiver , RX ) takes place via the pair of conductors only in one direction, the transmission is unidirectional . An input resistance of at least 4 kΩ is required on the receiver side  . There can be up to ten such receivers between the transmitter and the terminating resistor.

The EIA-485 interface is closely related to EIA-422 . There the line driver must have another input with which the output can be switched to high resistance , which enables half-duplex operation on one line, and the operating range against signal ground has been increased from ± 7 to −7 to +12 volts.

Overview of the most important specifications
parameter value
Number of senders / maximum number of receivers 1/10
Maximum cable length 1200 m
Maximum data transfer rate 10 Mbps
Maximum output voltage of the transmitter ± 6 V
Maximum differential voltage of the transmitter 10 V
Maximum short-circuit current of the transmitter 150 mA
Minimum differential voltage of the transmitter 2 V
Maximum voltage offset at the receiver ± 3V
Maximum voltage difference for both logic states 0.4V
Common mode input voltage ± 7 V
Minimum input resistance of the receiver (1 unit load) 4 kΩ
Input sensitivity of the receiver ± 200 mV
Minimum dielectric strength of the receiver 12 V (differential voltage)
Termination resistance 90 Ω - 150 Ω

Sender and receiver

polarity

An EIA-422 point-to-point connection

It always leads to confusion as to which of the two connections is the positive and which is the negative. The specification also does not include a negation circle on the circuit diagrams. However, this is often adopted by the EIA-485 as it is used synonymously. The specification only defines how the states are to be named on the transmission line. A '1' (MARK, OFF) is used when A is negative compared to B. If A is positive compared to B, a '0' (SPACE, ON) is used. These '1' and '0' states do not have to have anything to do with those at the digital inputs and outputs of the transmitter and receiver. EIA-422 does not define any logical function; this is viewed as application-specific, so the sender and receiver can contain a negation , for example .

EIA-422 transmitter

EIA-422 transmitter

The transmitter usually converts a digital input signal with TTL levels into a signal of the same polarity and a signal of opposite polarity. In principle, this can mean that a signal with negative voltage is output, in practice it only means that the input signal is negated. The non-inverting output is designated A (often Y), the inverting output B (often Z).

The two signal lines form a closed circuit with the terminating resistor (if present) and the input of the receiver. The current through this circuit is constant except for the switching torque and therefore hardly disturbs the own supply. In addition, in contrast to EIA-232, the ground line is not stressed by the transmission.

EIA-422 receiver

EIA-422 receiver

The outputs A and B of the transmitter correspond to the inputs A '(often A) and B' (often B) of the receiver. In practice, the logical function of the receiver is to convert the difference signal between A 'and B' into a digital signal with TTL levels. Analogous to the transmitter, A 'is the non-inverting input and B' is the inverting input.

The range between +200 mV and −200 mV is not defined. With many receivers, these switching thresholds are provided with a hysteresis in order to improve signal detection. The large threshold value allows the use of fast CFAs (current-feedback amplifiers), which are constructed with NPN and PNP transistors and, in contrast to VFAs (voltage-feedback amplifiers) from z. B. only NPN transistors (see differential amplifier ) have a larger offset.

The specification of the receiver is identical to that of the EIA-423 receiver.

recommendations

Different terminations: simple, RC and failsafe termination

Termination

The line should be terminated for transmission rates of 200 kbps or more. For this purpose, a resistor adapted to the impedance of the line is connected at the end of the transmission path . Optionally, the termination resistor can be connected in series with a capacitor ( RC termination ). This reduces the power dissipation of the circuit, since the termination resistor is only active dynamically. However, the maximum transmission rate and the cable length are limited. Another termination option is failsafe termination. Here, one signal line is provided with a pull-up and a pull-down resistor, which ensure a defined level in the event of failure of the transmitter, a cable break or an open receiver.

Cable length vs. Data rate

Transfer rate

The possible transfer rate depends on the cable length. The maximum transmission rate of 10 Mbps is only possible with a cable length of approx. 12 m, with the maximum line length of 1200 m only a maximum transmission rate of approx. 90 kbps is possible. These are guidelines; the values ​​can be significantly improved by choosing a suitable transmission line and circuitry.

Other Recommendations

In order to ensure the best possible data transmission, certain things are recommended and absolutely necessary from certain data rates.

  • In order to impair the environment as little as possible through electromagnetic interference (EMI, see also Electromagnetic Compatibility ), the respective line pairs should be twisted (UTP: unshielded twisted pair ). The resulting magnetic field rotates along the line and is almost canceled out as a far field. Conversely, this also makes the cable more robust against external EMI.
  • The mechanical design of the line and the material properties determine the electrical parameters of the transmission channel. A very important parameter is the wave impedance . The transmission channel should be closed on the receiver side so that an optimal power adjustment is guaranteed with minimal reflection. To do this, an (ohmic) resistor, also known as a terminating resistor or terminator, is connected in parallel, that is, between the corresponding signal lines, in terms of value approximately as large as the wave impedance.
  • At high transmission rates, the homogeneity of the transmission channel plays a major role. Are there jumps in impedance on the line, e.g. B. by unsuitable plugs or by extension with another line, disturbing reflections arise at the transition points. The propagation is similar to that of the useful signal in waves, but in both directions. On the receiver side, reflections lead to reading errors, in the worst case to signal cancellation.
  • If even higher demands are placed on the transmission (due to higher transmission rates), additional capacitive shielding in the form of a jacket is required ( shielded twisted pair (STP), e.g. CAT3 to CAT5 cable for Ethernet)

See also

Individual evidence

  1. ↑ Recommendation V.11 (10/96)

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