Theory Of 2 And 4-Wire Rs-485 Operation - TC Communications TC1540 User Manual

Theory of 2 and 4-Wire RS-485 Operation

Two-wire RS-485 is a half-duplex operation, which means transmit and receive operations take turns; they
do not function concurrently. From the user's point-of-view, the channel is in receiving mode when no data
is transmitted from the remote unit.
Four-wire RS-485 is a full-duplex operation, which means transmit and receive operations function
concurrently. From the user's point-of-view, both channels are in the receiving or transmitting mode at all
times.
The RS-485 Driver/Receiver transition time is determined by three DIP switches on the TC1540. The local
unit's RS-485 transceiver changes to transmit (Tx) mode upon receiving the first bit of data from the remote
unit. The transition time is the amount of time before the local unit's RS-485 transceiver will revert to the
high-impedance receiver mode after the last bit is received from the remote unit. The RS-485 transition time
should be set according to Table 1 on the following page.
Fiber Cable
Figure 5. 2-Wire RS-485 Logic Diagram
Fiber Cable
Figure 6. 4-Wire RS-485 Logic Diagram
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TC1540D User's Manual
Rev. 1.2