Hot answers tagged

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.


5

It doesn't matter as even if you're using a USART (which provides an additional CLK pin for synchronous operation) you'll only be using the UART part (RXD/TXD). Keep in mind that you'll need an additional pin for the Data Enable signal.


4

To be clear I don't understand wireless the ways the engineers here do, and that may be the best option for you. When you're evaluating wireless though, take into account battery cost, effect of temperature on battery life if you have high or low temperatures there and impact on the old envirothingy. Bear in mind wireless communication and wired power is ...


4

E0 is very typical of a baud-rate mismatch, in which the transmitter is sending at 1/6 the speed that the receiver is expecting. In your case, 9600 bps vs. 57600 bps. You see, when you transmit 55, the actual bit sequence is ...1110101010101111..., where the initial ones are the "idle" line, the first zero is the "start" bit and the next 8 bits are the data,...


4

Normally trace width and separation is adjusted so that the traces will have the necessary characteristic impedance, so it looks like another transmission line just like the cable does. If the PCB traces are not very long then the mismatch is minimal even if impedance is not matched. Some connectors have controlled impedanve but not all, in this case the ...


4

Any communications channel that is multiplex means it can be accessed by more than one transmitter. That's pretty general; in your context it means that different transmitters can use it at different times. A bus master is the piece of hardware that actually controls who has access to the bus. So a multi-master bus is one that has a mechanism for multiple ...


4

Is there any risk of damaging chips if one is sending a high bit (0v because it's UART) when others are staying high (idle or low bit)? My understanding is that multi-master RS-485 (at least one possible scheme anyways) has every node also has its receiver enabled when transmitting so it can monitor what is actually being put onto the line. If whatever ...


4

In short: A ground is not needed, because of RS485's characteristics. Isolation is in most cases best. TIA/EIA 485 specifies that no more than ±7V of potential between the grounds of two separate devices (shown as GPD below). The correct method for designing a differential data link is without ground wires. For ground potential differences higher than ...


4

It is probably due to the fact that you have not connected up the VCC and GND to the two modules. The initial communications transitions are back feeding the VCC and GND through the input protection diodes of the MAX485 until the C1 bypass capacitor gets charged up to a stable value. Make sure not to run your RS485 transceivers without power applied.


3

If you only want to receive, then yes, shorting both DE and RE to ground is the way to go. According to the datasheet: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime In other words, you ...


3

Classically, if you can HIDE the reflections within the rise and fall times, then you have a robust system, IF you have some hysteresis. With 4 microseconds SYMBOL (NRZ, at that) time, will you set up the link for 3 microseconds Rise and Fall times? and hysteresis at 25% and at 75% of the full scale swing?


3

RS485 clearly specifies a network topology without stubs, well except ones few milimeters from connector to the transceiver IC. So it is up to you to decide. I would like to suggest you to use additional 2x6m = 12m of cable and eliminate the stubs, afterall 250 kbps isn't a slow speed.


3

D28 and D27 are clamping diodes; whenever either of the data lines sinks below ground, the diode will source current into it to clamp it back to ground. The polarity of the diodes here is correct. Note that because RS-485 is designed to operate with a wide common-mode range these diodes will wrongly clamp the signal when common mode dips below ground (as ...


3

The header should include also size of data packet and everything should end with CRC or some checksum. You say that you have been inspired by Modbus, so why you didn't copy the whole protocol, which runs perfectly over decades in industry? If you include checksum at end, there is no way the telegram can be wrongly interpreted


3

Not really. All you've done is connect one of the differential inputs to ground and you're using the other input to read a single-ended signal. Since you have the SN75176A transceiver, just use another one to convert the test signal to proper differential form.


2

SHort Answer: You must add the differential terminators (120//120) for Rx and Tx combined OR use the Fig 6 sol'n for the MAX3095. RN7 is redundant. In the static case with uC outputs defined as inputs, U35 will detect the open circuit with its internal 47k pullup and force a logic high output. Meanwhile U4 will always sees this differential Logic High ...


2

Without going into too much math, I'd say keeping the stub length shorter would be better because it would reduce transmission line effects: Data transmission lines should always be terminated and stubs should be as short as possible to avoid signal reflections on the line. Proper termination requires the matching of the terminating resistors, RT, ...


2

50 slaves sounds a lot like you want a proper network with flexible architecture instead of a shared bus, even if only to isolate problems. Your rather complex graph illustrates how little a daisy-chained bus would work for you. As such, Ethernet is pretty popular, and mature. With IP and TCP atop of that, there's even proven software stacks that you can ...


2

U27a receive and U29a receive are isolated by R123 680 ohms. MPU should have control but this addition implies that both receivers may be active at the same time. In that case U27a will always dominate over U29a. If U27a is NOT in receive mode (it goes to high impedance if RE\ is high) then R123 allows data from U29a to be read. A simple and crude priority ...


2

1) Those RS485 drivers are bidirectional. They have a transmit and a receive circuit inside. That is why you see the two pins DI (driver input) and RO (receive output). 2) It depends on the data rate and/or the distance between the nodes. That is, for long runs of cable, you may need it, same it's true for high data rates. So I would put those there just in ...


2

If the Wikipedia article on J1708 is to be believed, then it's only J1708 if it's going at 9600 baud. You may be able to make it work over short distances at higher baud rates, but I doubt you'll be able to make it work over J1708's specified 40m distance -- and regardless, it wouldn't be J1708 any more.


2

Only single-ended, unidirectional signals can be terminated using a series resistor at the driver. RS485 is balanced so termination is different. The resistor goes between the balanced pair at the receiver for point-to-point. For multi-point, you daisy chain and terminate at each end of the daisy chain (not stubs) if the driver is not sitting there. The ...


2

The RS422 and RS485 standards only specify the electrical requirements. Both can be full-duplex but isn't a requirement of either specification. They are also similar enough that you can interoperate them in some situations. Now the big difference, which you already noted is that RS485 uses a tri-state system. This means you can have multiple transmitters ...


2

The TVS between each line to ground limits how far away each line can get away from ground (it clamps the common mode voltage). The TVS between the lines prevents the lines from getting too far away from each other (it clamps the differential voltage). In a way, the TVS from each line to ground does indirectly prevent each line from getting too far away ...


2

The goal of any ESD design is to shunt the unwanted current back to the source or to earth. If an ESD event happens on RS485 cables, the best thing to do would be to shunt it to earth, which in most designs is through the PCB ground, then chassis ground. Because RS485 is differential, if there are TVS diodes to ground on both lines then then any common ...


2

RS485 is purely a physical layer specification. You need your own protocol for anything you run over it eg home made, or something like MODBUS. Normally I would have just one master that requests information from connected devices. For anything multimaster or more complex the use a different bus eg CAN or Ethernet


2

I need to figure out the communication bus type used in it. As commented, those ICs are only used for RS-232 (they cannot be used for RS-422 or RS-485). Your CPU card might have other interfaces too, of course, but RS-232 is the answer about what interface type those ICs are used for. Here is an example - an old IBM PC-compatible serial card, showing ...


2

Yes, you do want to keep the trace impedance on the board as close to 120Ω as you can, mainly because this affects how the signal propagates on the cables attached to it. Presumably these devices are being daisy-chained together, and the reflections caused by impedance discontinuities will accumulate to reduce your overall signal integrity. There are ...


2

-7V to 12V is the tolerable common mode voltage of the differential line, not the signalling voltage on that line (and definitely not the signalling voltage on the microcontroller side). It's not directly related to the system voltage. You should also be asking if a 3.3V transceiver can interface with a 5V transceiver. (Yes, yes it can). It's a bit of a ...


2

A real RS485 bus would be setup for bidirectional transmission and not dedicated to one way flow. If you want to use unidirectional flow you should be more apt to study termination techniques for RS422. Symmetrical parallel termination at each end like you show is what you want for a bus that needs to transmit in both directions. For unidirectional ...


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