B&B Electronics 3PCIU4 - Manual (Page 68)

RS-422/485 Connections/Operation
62 Chapter 8 Manual Documentation Number 3PCIoUx-1008
Figure 45. An RS-485 Two-Wire Multidrop Connection
Send Data Control
MIport cards provide Send Data Control (SDC) for the RS-485 driver and
receiver. This is hardware controlled based on the contents of the UART
output buffer. When data is present, the driver is enabled; when the output
buffer becomes empty, it is disabled. This automatically handles whatever
baud rate is used.
The RS-485 transmitter and receiver have separate settings for 2-wire modes
(TX SD, RX SD) or 4-wire (TX SD, RX ENABLE).
Figure 46. Setting RTS Control for RS-485