The EIA-422 Standard
The EIA-422 defines a standard for serial communications. EIA-422 is a high speed or long distance transmission method. EIA-422 system software differs little from familiar point to point EIA-232 communication systems. EIA-422 is often used to extend the distance between nodes over the capabilities of EIA-232. When communicating at high data rates or over long distances in real world environments, single ended methods are often inadequate. Differential data transmission (balanced differential signal) offers superior performance in most applications. Differential signals can help nullify the effects of ground shifts and induced noise signals that appear as common mode voltages on a network. EIA-422 is also specified for multi-drop (party-line) applications where only one driver is connected to, and transmits on a ”bus” of up to 10 receivers. While a multi-drop “type” application has many advantages, EIA-422 devices cannot be used to construct a truly multi-point network. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit and receive. However a “Quasi” multi-drop network using EIA-422 consisting of 4 wires can enable a single master in a system to send a command to one of several slave devices on a network. The master node simply prefixes the commands with the appropriate address of the slave. There is no data echo or turn around delays to consider. Since each of the slaves transmitters shares the same pair of wires, care should be taken that the master never requests data from multiple nodes at the same time or data collision can occur.
Specifications | EIA-422 |
| |
Mode of operation. | Differential |
Total number of Drivers and | 1 driver, 10 receivers |
receivers on one line | |
Maximum Driver Output | -0.25V to +6V |
Voltage | |
Driver Output signal loaded | +/-2.0V |
Driver Output signal unloaded | +/-6V |
Driver load Impedance | 100 |
(Ohms) | |
Slew Rate | fast/slow |
Receiver Input Voltage range | -10V to + 10V |
Receiver Input Sensitivity | +/-200mV |
Receiver Input Resistance | 4k min. |
(Ohms) | |
| |
UltraPort Serial Adaptors User Guide
The EIA-422 Standard
Page 134
