RS–422/485 – 17
RS-422/485
4 Wire (Full Duplex)
Transmission
T+
T-
R+
R-
G
T+
T-
R+
R-
G
T+
T-
R+
R-
G
Host
Node 1
Slave
Node 2
Slave
Node 3
Slave
Node 4
T+
T-
R+
R-
G
Figure 3–2—Four Wire Communication
Transmission Timing
The transmitter must be enabled prior to transmitting a message
and disabled immediately after the stop bit of the last character of
the message has been sent. The transmitter of the RS–422/485 is
enabled/disabled using the DTR control for the associated port.
The register for the DTR signal is located at address Base + 24H
for P2 and Base + 34H for P3. Bit 0 of the register must be cleared
to a 0 to enable the RS–422/485 transmitter.
After the last byte of the information is sent, the transmitter must
be disabled. Before disabling the transmitter, make sure the
following conditions exist:
•
The UART transmitter holding register is empty.
•
The UART shift register is empty.
To disable the transmitter, set bit 0 of the register to a 1. This
algorithm works for both two and four wire, talker and listener
modes of operation.
NOTE: When modifying DTR, do not modify other bits in the
register.
Summary of Contents for 5554
Page 8: ...ii ...
Page 12: ...Preface 4 This page intentionally left blank ...
Page 14: ...Overview 6 This page intentionally left blank ...
