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Chapter 6
Documentation Number 3PCIO1-0903 Manual
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&B Electronics 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 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 section on RS-485.
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 3PCIO1 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
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(Windows 98 shown)
