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Hot answers tagged rs485

38

It signifies nothing; it's part of a sequential list of EIA standards: -

27

It's the document serial number of the standard. Same reason why the HTTP protocol is also known as RFC2616 and the Javascript programming language is also known as ECMA262. The numbers themselves have no meaning. For example while EIA232 specifies the electrical characteristics of a digital serial communications system, EIA222 specifies standards for ...

22

Welcome to the biggest challenge with half-duplex communications systems. RS-485 is not a protocol, it's a standard which defines the electrical properties for a half-duplex(*) differential link. There is nothing in the specification about how data is to be sent over that link, or in fact how the link is used. As such RS-485 transceivers have no automatic ...

12

While often shielded, CAT5 can be of the UTP type, meaning unshielded twisted pair. The "true" RS-485 cable you link to has two twisted pairs and a shield. If I remember right, CAT5e (and above) has a shield, at least most cables I have seen have - the exact standard can vary. I guess those would do the job just nice. Just make sure you use the twisted pair ...

12

RS-422 and RS-485 use the same electrical interface specification. If you use that interface to build a bidirectional link on a single pair of wires, it's called RS-485, and it supports half-duplex operation. If you use that interface to build two unidirectional links on two pairs of wires, it's called RS-422, and it supports full-duplex operation. This is ...

12

Yes. All almost ICs need decoupling capacitors. Devices such as these '485 drivers especially need them due to the current surges the device experiences when switching the signal states to the low value termination resistors used on the bus lines.

10

Driver Chips and DMX bus termination When searching the internet you can see several chips being used to drive a DMX communication. They all are made to convert logic-level signals (LOW & HIGH) to the EIA-485 differential signal levels on the twisted pair cable. Very common are the chips from MAXIM that I also use for my implementation. The one I ...

9

In general, for short cables (< 20-30m) and low baudrates (< 115200) you can leave them out without much trouble. But: It is useful to put some kind of load on the signal lines to improve noise immunity (the RS485 driver will supply enough current to switch the voltage on the differential line, many noise sources will not). But you do not need this ...

9

You don't need any pull-up or pull-down resistors if you're driving those pins with normal output pins on your micro. DE is the 'Driver Enable' pin and must be pulled high while you're transmitting data. Depending on your micro and how you're using its interrupts you may need to be careful about when you pull it back low - check that all of the bits are ...

9

It's very similar to radio communication of the military or the police. A protocol is required. Master slave is easy and good for most cases. But another option is to do it like humans do: Listen. If someone speaks- wait. If you think no one speaks- you can speak. Wait for confirmation. If no confirmation received- speak again. If you want to broadcast,...

8

All the ways you suggested will work to some extent, but some sort of computer based device is liable to be best . RS485 uses 2 wire differential voltage signalling and will usually have enough drive to allow an extra receiver to be attached. "Monitor" is probably a better term than "hijacker" :-). A computer will provide the most flexible result as you ...

8

More than likely it switches into transmit mode when TXD becomes active and a diode and capacitor "latch" that first transmit edge and hold that voltage "active" for longer than at least one byte. I see a little surface mount diode on the PCB (D1) and quite possibly this is what I'm referring to. I have seen this trick done before and the only disadvantage ...

8

When comparing the physical layer only, CAN and RS-485 are similar in that they both use differential signaling. This gives them both good common mode noise immunity. The main difference is that RS-485 uses symmetric signaling. One line is 5 V and the other 0 V to signal one state, then flipped to 0 V and 5 V for the other state. This makes detecting the ...

8

Looks like a common mode voltage problem to me. Although differential signalling, it still needs a common GND. (source) Replace your 1nf 500v capacitor with a jumper. Or wire signal ground to all nodes. Shield still stays connected to earth. The shield is connected to the Chassis ground on the various devices in the loop - and not at the 'master' end - ...

7

I want to know if there is anyway I can combine the serial protocol communicating via the RS485 interface RS485 doesn't have any defined serial protocol -- it's just the physical layer for specifying a multidrop differential signaling network, + a UART data link layer for encoding bytes via start/stop bits like RS232.

7

I would think it would be asking for trouble if you did not use termination and grounding. See Jan Axelson's write up. Also the further your cable extends, the lower the baud rate will be. At 4000' the maximum baud rate is 90kbps. I think using the shield as the ground is okay, but you need to isolate each node with 100 ohm resistors. I have used RS485 for ...

7

RS-485 and RS-232 a electrical-only standards. The same data encoding scheme is usually used on both. The difference is that RS-232 uses a single wire for the signal with levels below -5 V and above +5 V, while RS-485 uses a differential pair with 0-5 V levels. RS-485 is also intended to be multi-drop whereas RS-232 is point to point. You can chose to ...

7

I suspect the answer is in bold in the first line of your question. The resistor is between the data lines but the differential voltage is only a few volts - the common mode voltage can be anything but won't cause an increase in potential across the termination resistors. Figure 1. Extract from Linear's TIA/EIA-485-A Standard. The table above says that the ...

6

Oh, the reflections are there, don't worry :-). It's just that for short distances the delay of the reflected signal is so short that it won't distort the original signal much. Think of a 1 µs pulse, in a typical cable that will stretch over 200 m (at 2/3 $c$). Then at a 2 m cable the reflection will come after 1 % of the pulse width, and the ...

6

Generally speaking CAT5 s fine for RS485. IME the first limit you hit is the series resistance driving a termination over a long cable. I've run 250kbaud over 100m reliably. Things started getting shaky at around 200-300m.

6

Since you like parts from Maxim now, take a look at the Maxim MAX3160 and friends. They provide transceivers for both RS-232 and RS-485 protocols, allowing the protocol to be chosen at run time by driving a logic level input. We are using the MAX3161 in a couple of projects which provide only a three contact terminal strip for field wiring the serial cable (...

6

You can find the required voltage difference at the sending and receiving ends in the RS485 standard. As a second best, read the specs of some RS485 transceiver. What is probably the problem in your project (but it is difficult to reconstruct the original signal after a half-garbling filter) is that the common of your sender and receiver differ too much. ...

6

If I had to design a product that needed to be able to connect to either CAN or RS-485, I'd probably start by considering separate connectors or separate pins on the same connector for the two busses. The ground pin can be common if on the same connector. Jumpers can be made to work, but users don't always set them right, especially if multiple jumpers ...

6

It is a myth that you can make RS485 interfaces work without the Common (C) ground wire connected up between the various devices on the bus. The receiver is only capable of measuring the relative potential between the A and B signals when the common mode voltage of the A and B inputs is kept within -7V to +12V of the GND reference of the receiver. The idea ...

6

According to MAX485 datasheet, for DIP/SOP packages the ground goes to Pin5, and +5Vcc goes to Pin8. You seem to have the chip powered upside-down.

5

It's due to the 16ms latency timer of the FTDI driver, and the fact that my polling responses were not long enough to fill the 64-byte buffer to automatically trigger the buffer emptying. Read AN232B-04_DataLatencyFlow.pdf if you are interested, or simply go to your Device Manager, and change the settings in your USB-Serial-Port properties.

5

What's the data rate of your RS-485? Since your cable isn't twisted pair you may have to limit the speed to 100kbps and use short cables. But IMO the mismatched impedance isn't the real problem. "The characteristic impedance of an RS-485 cable is 120 ohm according to the standard, but cables designed for high frequency operation usually has an ...

5

"$D_{OUT}$ (Pin A1): General Purpose Logic Output. Logic output connected through isolation path to $D_{IN}$ . Under the condition of an isolation communication failure $D_{OUT}$ is in a high impedance state." So this is not a copy of the received signal, but a loopback from the input it gets from your microcontroller. In normal operation ...

5

An EIA-485 ("RS" refers to "Recommended Standard", and is an obsolete name) interface is rare on a PC, you'll need a separate interface board for that. The reason is that EIA-232 and EIA-485 are not hardware compatible, despite using the same connector type: EIA-232 uses single-ended signals, meaning that RxD and TxD are referenced to ground. EIA-485 uses ...

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