Impulse 3440 - Manual (Page 9)

Card Setup
Sealevel Systems ULTRA COMM+422 Page 6
RS-485 Enable Modes
RS-485 is ideal for multi-drop or network environments. RS-485 requires a
tri-state driver (not dual-state) 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 COMM+422 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 COMM+422 in an RS-485
application with existing (i.e. standard RS-232) software drivers.
Headers J1D through J4D 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 J1D through J4D 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’.
For RS-422/530/449 compatibility remove the jumpers at J1D through J4D.
Examples on the following pages describe all of the valid settings for J1D - J4D.