RS-422

EIA-422, normalized as RS- 422 and in Europe as the ITU- T V.11, is an interface standard for a 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 ) ".

• 3.1 scheduling
• 3.2 Transfer Rate
• 3.3 Other Recommendations

Description

EIA-422 only specifies the electrical characteristics of the interface, it does not define a protocol and no pin assignment. It is envisaged that the EIA-422 specification is referenced by other specifications. Therefore, there is no uniform pinout of EIA -422 connector, so that when using different EIA-422 devices, the documentation of the device must always be consulted.

In contrast to the single-ended serial port according to standard EIA- 232, EIA -422 is designed for balanced signal transmission. This means that from the transmitter to the receiver in each case a positive and a negative signal is transmitted through a respective wire of a twisted wire pair. This common-mode disturbances are minimized and at the same time achieves higher data rates.

With EIA-422 can be built full-duplex point-to -point connections, but also multi- drop networks are possible. In the latter up to ten receivers can be connected to a transmitter. The transmission from a transmitter (English transmitter, TX ) to a receiver (English receiver RX) is done via the line pair in one direction, the transmission is unidirectional. Accordingly you need for a bi-directional data transfer in addition to the common ground at least four lines: In each case one line pair for one direction. If you want to a hardware handshake similar use as EIA -232, according to another line pairs are required.

Due to the higher data rates up to the range of a few Mbit / s, the line pairs an EIA-422 interface on the receiver must be terminated with a terminating resistor of typically 120 ohms. Accordingly, the receiver is adapted to (from ± 200 mV) to reliably detect small difference voltages. At a point-to -point connection as in the EIA-422, the voltage drop across the line hardly plays similar to stream interface, a roll, only the voltage drop across the termination resistor, close to the receiver.

Closely related to EIA-422 is the interface to EIA - 485th This differs only slightly in the electrical parameters of which the EIA -422 drivers. Mostly, the on the market today EIA -422 driver blocks can be used for both EIA-422 and EIA -485. EIA- 485 enables the construction of extended networks; hence multipoint networks with multiple transmitters and receivers are possible. EIA- 485 interfaces are therefore often incorrectly referred because of the similarity to EIA-422 in practice as EIA- 422 interface, although they are not 100% compatible.

A derived from EIA-422 protocol is VDCP.

The transmitter and receiver

Polarity

It always leads to confusion, which of the two terminals of positive and negative which the. The specification does not contain a negation circle on the wiring diagrams. This but is often taken over by the synonymous use with EIA- 485 of this. The specification only defines how the states are to be named on the transmission line. It is spoken by a '1 ' ( MARK OFF ) when A is negative with respect to B. If A is to B positive, it is spoken by a '0 ' (SPACE, ON). This, 1'- and '0 ' states must have nothing with which to do on the digital inputs and outputs of the transmitter and receiver. EIA-422 does not define a logical function; this is considered to be application specific, so the transmitter and receiver may comprise, for example, a negation.

EIA -422 transmitter

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

The two signal lines form with the termination resistor ( if present ) and the input of the receiver has a closed circuit. The current through this circuit is constant up to the Umschaltmomente and thus hardly disturbs its own supply. In addition, the ground line is not loaded by the transmission unlike EIA-232.

EIA-422 receiver

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, the implementation of the differential signal between the A 'and B' into a digital signal at TTL levels. It is analogous to the station A ', the non-inverting input, and B', the inverting input.

The region between 200 mV and -200 mV is not defined. In many receivers these thresholds are provided with a hysteresis to improve signal detection. The large threshold for production reasons. Fast amplifiers are as CFA (current feedback- amplifier ) built and, in contrast to VFAs (voltage feed back amplifier ) due to different characteristics of NPN and PNP transistors, a larger offset. VFAs have a balanced input stage consisting of the same technology (eg NPN, see differential amplifier ), and therefore a smaller offset ( or here called threshold ).

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

Recommendations

Termination

With transfer rates of 200 kbps, the pipe should be terminated. To one of the impedance of the line is connected at the end adapted resistance of the transmission path. Optionally, the terminating resistor may be connected in series with a capacitor (RC termination). This reduces the power dissipation of the circuit because of the termination resistor then only dynamic effect. In this case, however, the maximum transmission rate and the line length are limited. Another scheduling option is the fail-safe termination. Here, a signal line each provided with a pull-up and pull- down resistor that ensure failure of the transmitter, cable breakage or open receiver for defined levels.

Transfer rate

The transmission rate depends on the cable length. The maximum transfer rate of 10 Mbps is only possible with a cable length of 12 m, at the maximum cable length of 1200 meters and only a maximum transfer rate of about 90 kbps is possible. These are indicative and by appropriate choice of the transmission line and the values ​​of the circuits can be significantly improved.

Other Recommendations

To ensure the best possible data transmission, certain things are recommended and from certain data rates necessary.

• To set the environment as little as possible to electromagnetic interference: to affect (English " electromagnetic interference ", EMI, see also Electromagnetic compatibility), the respective pairs of wires should be twisted pair (UTP: unshielded twisted pair ) to be. The resulting magnetic field rotates so along the line and stands out as the far field almost on. Conversely, as the line also more robust against external EMI.
• The mechanical design of the line and the material properties determine the electrical characteristics of the transmission channel. A very important characteristic is the wave impedance. The transmission channel should be completed on the receiver side for optimum power matching is ensured with minimal reflection. These switches are a ( ohmic ) resistance, also known as Terminator or Terminator, in value to about the size of the wave impedance, in parallel, that is, between the corresponding signal lines.
• At high data rates the homogeneity of the transmission channel plays an important role. Is it on the line impedance jumps, eg by using unsuitable plugs or extension to another line, so annoying reflections occur at the transition points. The propagation is similar to the desired signal wave-shaped, but in both directions. On the receiver side reflections lead to read errors, at worst, a signal void.
• If even greater demands placed on the transmission ( because of higher transfer rates ), a further capacitive shielding is still in the form of a jacket is required ( STP [ shielded twisted pair ] eg CAT3 to CAT5 cable for Ethernet)
• Since no electrical isolation exists, it is possible to transfer both NRZ and RZ. The lack of isolation also has a disadvantage. The receiver can determine the difference signal correctly only if it is within the supply voltage range of the receiver ( 5 V; currently rather 3.3 V). In order to avoid too large potential shifts, must always be accompanied by a signal ground ( via the screen the signal ground to connect with each other is only a temporary solution ).