RS-422/485 Application Note 9
© Copyright B&B Electronics -- Revised 2506
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
An RS-485 network can also be connected in a four-wire mode as shown
in Figure 1.7. Note that four data wires and an additional signal ground wire
are used in a “four-wire” connection. In a four-wire network it is necessary
that one node be a master node and all others be slaves. The network is
connected so that the master node communicates to all slave nodes. All slave
nodes communicate only with the master node. This network has some
advantages with equipment with mixed protocol communications. Since the
slave nodes never listen to another slave response to the master, a slave node
cannot reply incorrectly to another slave node.
Tristate Control of an RS-485 Device using RTS
As discussed previously, an RS-485 system must have a driver that can be
disconnected from the transmission line when a particular node is not
transmitting. In an RS-232 to RS-485 converter or an RS-485 serial card, this
may be implemented using the RTS control signal from an asynchronous serial
port to enable the RS-485 driver. The RTS line is connected to the RS-485
driver enable such that setting the RTS line to a high (logic 1) state enables the
RS-485 driver. Setting the RTS line low (logic 0) puts the driver into the
tristate condition. This in effect disconnects the driver from the bus, allowing
other nodes to transmit over the same wire pair. Figure 1.8 shows a timing
diagram for a typical RS-232 to RS-485 converter. The waveforms show what
happens if the VRTS waveform is narrower than the data VSD. This is not the
normal situation, but is shown here to illustrate the loss of a portion of the data
waveform. When RTS control is used, it is important to be certain that RTS is
set high before data is sent. Also, the RTS line must then be set low after the
last data bit is sent. This timing is done by the software used to control the
serial port and not by the converter.
When an RS-485 network is connected in a two-wire multidrop party line
mode, the receiver at each node will be connected to the line (see Figure 1.6).
The receiver can often be configured to receive an echo of its own data
transmission. This is desirable in some systems, and troublesome in others.
Be sure to check the data sheet for your converter to determine how the
receiver “enable” function is connected.
10 RS-422/485 Application Note
© Copyright B&B Electronics -- Revised 2506
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
Figure 1.8
(96 pages)
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Comments to this Manuals