Card Setup, Continued
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 485 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 485 in a RS-485 application with existing (i.e., standard
RS-232) software drivers.
Header E5 is used to control the RS-485 mode functions for the transmitter circuit. The selections are ‘RTS’
enable, ‘Auto’ enable, or ‘422’ which means always enabled. The ‘Auto’ enable feature automatically
enables/disables the RS-485 transmitter circuit. The ‘RTS’ mode uses the ‘RTS’ modem control signal to
enable the RS-485 transmitter circuit and provides backward compatibility with existing software products.
The ‘422’ mode allows the port to be used in a point to point RS-422 application where the tri-stating of the
transmitter circuit is not required.
The jumper in the above example is in the ‘422’ position. This is the only setting in which the modem
control outputs (RTS, DTR) are valid.
Figure 4 - Header E5 RS-485 Transmit Mode
