© Sealevel Systems, Inc. 7903 Manual | SL9072 7/2021
Line Termination
Typically, each end of the RS-485 bus must have line-terminating resistors (RS-422 terminates at 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. Switches SW1 and SW2 allow the user to
customize this interface to their specific requirements. Each switch position corresponds to a specific
portion of the interface. If multiple ISO COMM+2.cPCI adapters are configured in a RS-485 network, only
the boards on each end should have jumpers T, P & P ON. Refer to the following table for each position’s
operation:
T Adds or removes the 120 ohm termination.
P
Adds or removes the 1K ohm pull-down resistor in the RS-422/RS-
485 receiver circuit (Receive data only).
P
Adds or removes the 1K ohm pull-up resistor in the RS-422/RS-
485 receiver circuit (Receive data only).
L Connects the TX+ to RX+ for RS-485 two wire operation.
L Connects the TX- to RX- for RS-485 two wire operation.
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 ISO COMM+2.cPCI is the ability to be RS-485 compatible without the
need for special software or drivers. This ability is especially useful in Operating Systems where the lower
level I/O control is abstracted from the application program. This ability means that the user can
effectively use the ISO COMM+2.cPCI in an RS-485 application with existing (i.e. standard RS-232)
software drivers.
Switches SW3 and SW4 are used to control the RS-485 mode functions for the driver circuit. The
selections are ‘RTS’ enable (silk-screen ‘RT’ switch position 4) or ‘Auto’ enable (silk-screen ‘AT’ switch
position 3). 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.
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. Position 9 of SW1 and SW2 is used to control the RS-485 enable/disable functions for the receiver
circuit. To select the ‘No Echo’ mode place switch position 9 to the ‘On’ position (see Figure 5).
