How Fast can you Go?
RS-485 gives limits for rise and fall times as 0.3 of the unit interval. This will reduce the reflections on a RS-422 network to some extent, but RS-422 also recommends a termination resistor at the receiver’s end of the wires (well it sort of recommends, depending on the “data rate” or the “signal rise time at the load end of the cable”). In this implementation the on the wire format has been modified to use the Lego defined baud rate of 921600 bps and uses byte stuffing rather than bit stuffing (since there is no hardware support). There are no limits given in time units, RS485 standard so there is no minimum or maximum bit rate associated with RS-485. The receiver can monitor the wires and indicate a fault if a 0 is detected on the wires for an extended period of time. This object can be used to create new connections.
User connections are compression screw terminal strips for the RS485 ports, an RJ11/12 telephone style connector for the RS232 port, and a 2.1mm barrel connector or 2 pin terminal strip for supply power. The protocol has been integrated with the standard leJOS connection object and offers up to 7 connections. Again many thought that the UART protocol used in the serial port were part of the RS-485 standard, but the RS-485 standard contains even less of the OSI model layer one than RS-232 does. The RS-232 standard resides in layer one of the OSI model, or the physical layer. RS485 is used as the electrical layer for many well known interface standards, including Profibus and Modbus. This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. The RS-485 standard includes the voltage levels of the two wires when a binary 0 or 1 is on the two RS-485 wires, but specifically excludes the logic function of the generator or receiver. The serial port used a D-sub, B-shell, 25-pin connector (commonly called a DB-25) and RS-232 electrical signal levels, which made it “RS-232 compatible” or compatible with the RS-232 standard.
Disable the RS485 hardware port. The RS485 drivers feature enhanced ESD performance, and our design employs 22 Ohm series protection resistors. This scheme will automatically take control of the network when a byte is transmitted, but there will not be any guaranteed marking time so bias resistors may be required. The master RS485 port has jumper selectable bias and termination resistors. When used as a converter, the master RS232 port provides bi-directional serial conversion to the 8 RS485 distribution ports. Here the master initiates a communications request to a “slave node” by addressing that unit. In this situation, bandwidth can be used for almost 100%. There are other implementations of RS485 networks where every node can start a data session on its own. Look at the data sheets from each manufacturer to determine what signal is on what pin and if the two devices can be connected either directly or with an adapter. If this is required, it can be accomplished by any means desired including RS-485 drivers on an additional network, RS-232 drivers on extra wires, TTL levels, or really anything you can dream up since it won’t work with any other RS-485 network except the one you are designing.
The 4-wire design, as implemented by the C/MRI, takes advantage of both worlds, RS422 and RS485, by dedicating one set of the more advanced RS485 transceivers to sending data from the PC and a second set for handling data being received by the PC. Typical uses are very large networks, monitor and control of elevators, machinery, lighting, security, fire and alarm, access, cameras, military ground support, and much more. We provide free technical support, no-argument warranty repairs, and recommendations for your application. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Additionally, because RS485 was developed later than RS422, the RS485 transceiver ICs, a combination transmitter and receiver, exhibit higher input impedance and output drive capability. For example, using QuickBASIC with my Pentium in data acquisition, I read input bytes 200,000 times per second. Lines RB0-RB7 and RE0 perform as port enable lines to define which I/O port IC (U3-U5 for input or U9-U14 for output) is activated for each input and output operation over the I/O data bus connecting all the ICs to U1’s Port RD. Transmitting data bidirectionally over the same wire(s), rather than unidirectional transmission, requires a turn-around delay.
