If you need To be successful In RS485 Standard, Here are 5 Invaluable Issues To Know > 자유게시판

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The only way to make a cable resistant to higher temperatures (while keeping its mechanical properties somewhat normal) is by using heat-resistant plastics such as Silicone rubber or Teflon (PTFE). Note that PTFE is not halogen-free and Silicone seems to pose other health risks when on fire (ultimately is combustible, hard to extinguish, RS485 standard and develops a lot of nasty smoke when burning). This is (almost) no problem as long as the power is distributed along with the bus signals (note that it's inappropriate for other reasons to daisy-chain power distribution cabling just like the RS485 signals). There are devices that actually have such a properly isolated RS485 transceiver and DO NOT have a signal ground terminal. The fastest RS-485 transceivers out there right now are running about 50 megabits. On the scope I could see that the pulse shapes indeed got cleaner, but at the same time the pulse amplitude dropped quite a lot, and the transceivers probably didn't like that side-effect. If you have a nice-looking shielded twisted cable that doesn't have its impedance stated, you can find out yourselfs (see my reflectoprobe suggestions above) - you will likely measure something between 60 and 80 Ω diff-mode, and consequently 15-20 Ω common mode.

I haven't seen a dedicated RS485 cable made of those plastics. There are however lower-grade "fire-resistant" cables matching the general "shielded twisted pair" specification, for basic signaling applications, which can also be used for RS485 for shorter distances. If the pair impedance is stated, it'll almost certainly 100 Ω or 120 Ω or maybe something inbetween. At higher baud rates though, the lower Z means attenuation at the transmitter, compared to a proper 120 Ω cable (which translates into shorter reach). Optimum impedance for RS485 is 120 Ω (expected by the line drivers). Other impedance cable may be used such as 100 Ω nominal. There is a similar family of NOMAK cables which has only a common screen for all the pairs (individual pairs in the cable are unshielded). When the input of an RS-485 driver goes high, one of the outputs will go high in reference to circuit common and the other will go low. If the voltage on the RS-232 line is positive or high, the signal is a binary 0. Should this signal be inverted back to a low? For normal logic when the input of a driver is a 1 or high, the non-inverting output will go high.

The issue also exists in programmable logic controller applications. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. This is allright for basic applications (low transfer rate) and may actually mean longer reach (less DC resistance), as the lower RF Z typically means thicker conductors compared to insulation thickness (distance of conductors in the pair). If each pair is shielded (or there's only one signal pair in the cable), that's even better. If the cable doesn't have its nominal impedance stated, it's not per definition intended for high-performance data communications - but if it's at least shielded, it'll probably work just fine in a number of RS485 applications. If handshaking is required, it can be attempted using using X-On / X-Off handshaking protocol, but it is unlikely to work. Acuview allows for real-time monitoring, and remote metering access for PC based data logging using .xls, .csv, or.txt file formats. Devices with a properly galv.isolated RS485 port (isolated from PSU input) theoretically don't need a reference ground, and can work with the bus using just some weak internal biasing resistors.

There are a number of termination techniques, all of which may work great under a narrow range of conditions. In other types of communication gadgets, say a 232-to-485 converter, both the local (232) part and the long-reach (485) part may have to be powered somehow, usually by a miniature monolithic DC-DC converter. That way, the local data inteface (232 or 485) ends up being directly connected to the "brain" of the modem - the processor chip(s). The local 232 port will likely remain un-isolated from the converter's MCU, if there is any. When you hear that buzzword for the fist time, you will likely imagine three neat isolation gaps, surrounding the three ports (local data port, the longer-haul line interface, and the power input). It may become a problem when you need to power the IO gadget from a local source of power, that is not earth-isolated. For RS485, the impedance of just 70 Ω may prove a tad too low for longer distances. Quality STP cables for high-performace communications tend to have the nominal impedance stated in the datasheet (meaning the diff-mode impedance), often along with attenuation values in dB for a few radio frequencies.