3 Making RS-485/422 Connection
EIA-485[TIA-485] Balanced (differential) interface; defines the Physical layer, signaling protocol is not defined. EIA-485 specifies bidirectional, half-duplex data transmission. Up to 32 transmitters and 32 receivers may be interconnected in any combination, including one driver and multiple receivers (multi-drop), or one receiver and multiple drivers.
EIA/TIA-422define a Balanced (differential) interface; specifying a single, unidirectional driver with multiple receivers (up to 32). RS-422 will sup- port Point-to-Point, Multi-Drop topology, but not Multi-Point [EIA485]. EIA-485 devices may be used in 422 circuits, but EIA-422 may not be used in 485 circuits (because of the lack of an Enable line).
The published TIA/EIA 485 and RS-422 standards define only the electri- cal characteristics of the drivers and receivers as listed below. They did not standardize such things such as cables and connectors, pinouts, bus arbitration, signaling protocols, or physical wiring topology. Many dif- ferent implementations have come into use and they are often incompat- ible with each other.
Characteristics | RS-422 | RS-485 |
Mode of operation | Differential | Differential |
| Unidirectional | Full Duplex |
| Multipoint | Multipoint |
Allowed no. of Tx and Rx | 1 Tx, 10 Rx | 32 Tx, 32 Rx |
Maximum cable length | 4000ft length | 4000ft length |
Maximum data rate | 10Mbps | 10Mbps |
Minimum Tx driver output range | +/- 2V | +/- 1.5V |
Maximum Tx driver output range | +/- 5V | +/- 5V |
Maximum Tx short-circuit current 150mA | 250mA |
Tx load impedance | 100 | 54 |
Rx input sensitivity | +/- 200mV | +/- 200mV |
Maximum Rx input resistance | 4k | 12k |
Rx input voltage range | +/- 7V | -7V to +12V |
Rx logic high | >200mV | >200mV |
Rx logic low | <200mV | <200mV |