COMHP 7404/-D25
CyberResearch
®
Serial Communications
2
©Copyright 2010
CyberResearch, Inc.
Card Setup
RS-485 Enable Modes
RS-485 is ideal for multi-drop or network environments. RS-485 requires a tri-state driver that will allow the electrical
presence of the driver to be removed from the line. The driver is in a tri-state or high impedance condition when this occurs.
Only one driver may be active at a time and the other driver(s) must be tri-stated. The output modem control signal
R
equest
T
o
S
end (RTS) is typically used to control the state of the driver. Some communication software packages refer to RS-485 as
RTS enable or RTS block mode transfer.
One of the unique features of the
COMHP 7404
is the ability to be RS-485 compatible without the need for special software
or drivers. This ability is especially useful in Windows and other protected mode environments where the lower level I/O
control is abstracted from the application program. This ability means that the user can effectively use the
COMHP 7404
in
an RS-485 application with existing (i.e. standard RS-232) software drivers.
Switches SW1 - SW4 are used to control the RS-485 mode functions for the driver circuit. The selections are ‘RTS’ enable
(silk-screen ‘RT’) or ‘Auto’ enable (silk-screen ‘AT’). The ‘Auto’ enable feature automatically enables/disables the RS-485
interface. The ‘RTS’ mode uses the ‘RTS’ modem control signal to enable the RS-485 interface and provides backward
compatibility with existing software products.
Position 3 (silk-screen ‘NE’) of these switches is used to control the RS-485 enable/disable functions for the receiver circuit
and determine the state of the RS-422/485 driver. The RS-485 ‘Echo’ is the result of connecting the receiver inputs to the
transmitter outputs. Every time a character is transmitted; it is also received. This can be beneficial if the software can handle
echoing (i.e. using received characters to throttle the transmitter) or it can confuse the system if the software does not. To
select the ‘No Echo’ mode select silk-screen position ‘NE’.
Typically, each end of the RS-485 bus must have a line-terminating resistor (RS-422 terminates the receive end only). A
120-ohm resistor is across each RS-422/485 input in addition to a 1K-ohm pull-up/pull-down combination that biases the
receiver inputs. Only the ends of an RS-485 network should have the 120-Ohm terminating resistor. Position 4 (silk-screen
‘T’) selects the presence of the 120-Ohm termination resistor across the input data pins (RX+/RX-). To add the termination
select the ‘On’ position, to remove it select the ‘Off’ position.
For RS-422/530/449 compatibility set all switches ‘Off’. Examples on the following pages describe some of the valid
settings for SW1-SW4.