Rs-485 Enable Modes - SeaLevel SIO-485.LPCI User Manual

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.
This capability allows multiple PCs to be connected in a multi-drop bus and
selectively polled. Failure to correctly utilize the enable can cause transmitter
contention problems preventing operation by any node on the network. The SIO4-
104.485 utilizes the automatic RS-485 capabilities of the OX16C950 to control the
RS-485 tri-state enable. This is a highly efficient method of enable and allows for
minimum 'turn-around' times. If the Sealevel Systems Windows Software driver is
used, a 'Radio' style button on the 'Advanced' property page under the Device
manager can be selected that will configure the OX16C950 automatic RS-485 enable.
Another way of controlling the tri-state enable is with the output modem control
signal Request To Send (RTS). Some communication software packages refer to RS-
485 as RTS enable or RTS block mode transfer. The SIO4-104.485 is compatible
with this RS-485 mode as well. If the Sealevel Systems Windows Software driver is
used, a 'Radio' style button on the 'Advanced' property page under the Device
manager can be selected that will automatically toggle RTS for use as an enable.
Switch SW1 is used to control the RS-485 mode functions for the driver circuit. The
selections are:
'RTS' enable (silk-screen 'R') The 'RTS' mode uses the 'RTS' modem control
signal to enable the RS-485 interface and provides backward compatibility with
existing software products.
'DTR' enable (silk-screen 'D') 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.
Power Supply
The SIO-485.LPCI has the ability to provide DC power on pin 9 of the DB-9
connector to be used by the integrator in low power implementations. This power is
fused at 1A and is selectable as +5, +12 VDC or no power required.
© Sealevel Systems, Inc.
+12 E1 +5
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SIO-485.LPCI User Manual