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© Sealevel Systems, Inc.
7101 Manual | SL9160 9/2021
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 Request To Send (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 ULTRA 530.PCI is the ability to be RS-485 compatible without the need
for special software or drivers. This ability is especially useful in Windows, Windows NT, and OS/2
environments where the lower level I/O control is abstracted from the application program. This ability
means that the user can effectively use the
ULTRA 530.PCI
in a RS-485 application with existing (i.e.,
standard RS-232) software drivers.
Header E4 is used to control the RS-485 mode functions for the driver circuit. The selections are:
‘Auto’ enable (silk
-
screen ‘AT’). The ‘Auto’ enable feature automatically enables/disables the RS
-485
interface via on board circuitry.
‘RTS’ enable (silk
-
screen ‘RT’) The ‘RTS’ mode uses the ‘RTS’ mode
m control signal to enable the RS-485
interface and provides backward compatibility with existing software products.
‘DTR’ enable (silk
-
screen ‘DT’) The ‘DTR’ mode uses the ‘DTR’ modem control signal to enable the RS
-485
interface, provides backward compatibility with existing software products and with the Oxford
Semiconductor 16C950 RS-485 enable feature
‘Output One’ enable (silk
-
screen ‘OP’) This mode uses the ‘OP1’ control signal to enable the RS
-485 interface
and provides backward compatibility with existing software products and with the Exar 16C850 RS-485
enable feature
‘No Echo’ (silk
-
screen ‘NE’) of E4 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’ i
s 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)
o
r it can confuse the system if the software does not. To select the ‘No Echo’ mode select silk
-screen
position ‘NE’.