SeaLevel SIO-485.LPCI User Manual page 11

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.
+12 E1 +5
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© Sealevel Systems, Inc. 7107 Manual | SL9055 7/2021