DocumentationNumber 3PCI2-0903 Manual Chapter6 47
B&BElectronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&BElectronics Ltd – Westlink Comm. Pk. – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445
This limitation is due to the fact that the transmitter is active all the
time, and the line pair is set so that the TD(B) line is high during
Mark state, and the TD(A) line is set low. If another transmitter
output is connected to the same wire pair, and attempts to send data
by setting the line pair to Space state, the first transmitter is still
trying to hold the opposite state and neither can communicate. To
overcome this limitation, the RS-485 mode was established, where
each transmitter tri-states to a high impedance when not
transmitting. See following.
RS-485 Operation
In RS-485 mode the transmit driver must be enabled during transmit
(TX SD) and tri-stated to a high impedance when the data has been
sent. In the 2-wire (half-duplex) mode, the receiver is enabled when
not transmitting, and disabled (RX SD) during transmit (echo off).
The 3PCI2 provides Send Data (SD) Control 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).
Example - Settings, DTR Operation, RS-485 Mode selected.
(Windows 98 shown)