Table of Contents
Advertisement
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 OMG-ULTRA-485-PCI is the ability to be RS-485 compatible without the need
for special software or drivers. This ability is especially useful in Windows 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
OMG-ULTRA-485-PCI in an RS-485 application with existing (i.e. standard RS-232) software drivers.
Switch SW2 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' modem control signal to enable the RS-485
interface and provides backward compatibility with existing software products.
'No Echo' (silk-screen 'NE') 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'.
Clock Modes
The OMG-ULTRA-485-PCI employs a unique clocking option that allows the end user to select from divide by 4
and divide by 1 clocking modes. SW2 positions 1 and 2 select the Divide by 1 (silk-screen D1) and Divide by 4
(silk-screen D4) modes.
To select the Baud rates commonly associated with COM: ports (i.e. 2400, 4800, 9600, 19.2, ... 115.2K Bps ) set
switch SW2 in the divide by 4 mode (silk-screen D4).
To select the maximum data rate up to 460.8K bps set switch SW2 in the divide by 1 (silk-screen D1) position.
Please refer to the following page for these additional data rates.
Caution: Do not set both switches on or invalid clocking will occur.
RS-485 Mode and Clock Mode Examples (Switch SW2)
OMG-ULTRA-485-PCI
Card Setup
D
D
A
R
N
1
4
T
T
E
ON
OFF
1 2 3 4
Figure 1- Switch SW2, RS-422, Divide by 4
5
Card Setup
Page 2
Advertisement
Table of Contents
