The EIA-485 Standard
The EIA-485 standard is similar to the EIA-422 standard upon which it was based. The main difference is that up to 32 transmitter receiver pairs may be present on the line at one time. A 120-Ohm resistor integrated on the UltraPort SI card should be enabled to terminate either end of the main line. The UltraPort SI when configured for EIA-485 full duplex is compatible to use in a EIA-422 multi-drop environment. EIA-485 caan be used in situations where more than one device may transmit data on a shared data line. In general not all lines are used for EIA-485 systems. In a two wire half duplex configuration the TXD+ line is connected to the RXD+ while the TDX- line is connected to the RXD-, only one pair of twisted wire cable is used in a EIA-485 half duplex communications configuration. The special circuitry in the UltraPort SI card when set to auto will automatically sense when data is being transmitted and automatically enable the transmit line as well as disable the transmit line when the data has been sent. Some EIA-485 applications use the RTS handshake line to enable and disable the transmit line. In this case then the field EIA-485 HDX TX Control should be set to RTS. It is now the host application’s responsibility to enable and disable the transmit line via the RTS handshake line when it wants to send data. When the EIA-485 HDX TX Control field is set to AUTO, the UltraPort SI card allows you to further customized your application by applying a Auto EIA-485 Bit Delay. This field by default is setto 0 delay, however in some applications the user may want to increase this field up to 15 bits..
Specifications | EIA-485 |
| |
Mode of operation. | Differential |
Total number of Drivers and | 32 drivers or 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 |
HDX Control | +/-200mV |
Bit-delay | 4k min. |
| |
UltraPort Serial Adaptors User Guide
The EIA-485 Standard
Page 135
