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.
(Windows 98 shown)
46 | Chapter 6 | Documentation Number 3PCIO1-0903 Manual |
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