Filter
Top Products
UltraPort Serial Adaptors User Guide
The EIA-485 Standard
Page 134
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 set
to 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
receivers on one line
32 drivers or receivers
Maximum Driver Output
Voltage
-0.25V to +6V
Driver Output signal loaded +/-2.0V
Driver Output signal unloaded +/-6V
Driver load Impedance
(Ohms)
100
Slew Rate fast/slow
Receiver Input Voltage range -10V to + 10V
HDX Control +/-200mV
Bit-delay 4k min.