What are the implications of using a half-duplex serial connection versus a full-duplex one? What happens if both sides try sending data at the same time? Do you end up with corrupt data arriving on both ends? Does flow-control help you with this?
On the line the data will be garbled, which may or may not lead to devices receiving the garbled data. Sometimes this will be used to detect that the transmission failed due to a collision.
Normally you wouldn't use half-duplex in the same way as full-duplex to send single characters in asynchronous mode. Rather you'd use some packet protocol which determines who has the right to send at which times, and which includes some checksum (usually a CRC) to detect corruption.
Flow control doesn't help much for this. It's purpose is to ensure that the receiver is not overrun by to much data. There is software flow control which uses the ASCII characters XON and XOFF to start and stop transmission, and hardware flow control which uses the RTS (Request To Send) and CTS (Clear To Send) control lines. XON/XOFF-style software flow control won't work with half duplex.
These days you don't see half duplex with ordinary RS-232 and modems (I used it with acoustic couplers in the eighties, it was rare even then). But it is common for RS-485, which is used in industrial control with various protocols. There are also many other data transmission standards which operate in a half-duplex way, mostly when there are more than two devices attached to the same line (ancient 10base2 Ethernet, CAN, LIN, FlexRay, I2C, ...).
My God, where did you find a half-duplex line in this day and age?
Anyway, the answer is that if both ends drive the line, it gets all confused. For this reason, there are specified ASCII characters lie Clear to Send and Data Terminal Ready (CTS and DTR) that are used to make a handshake. See this tutorial for more.
Augh, I should have gone to bed. Tutorial right, me stoopid.
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I have severe hardware limitations on a school project. To add a feature we desire, I am limited to 2 bit communication between two micro controllers. They share two wires that can independently go low or high.
If I wanted to send the string "Hello World" hex encoded across the wire, what protocol would ensure the message arrived safely and with minimal loss?
I2C ( I squared C ) is a easy to bitbash synchronous protocol that has collision detection but requires open collector lines as multiple devices could be driving the lines at the same time.
Otherwise You can bit bash serial you just have to find a slow enough baud rate where it's reliable if you only have software delays to control timing with
On top of that then have a protocol something like
STX <DATA> ETX <CRC>
You can make this more complex as needed. For instance sometimes it's way nicer if you can say up front how big the message is <STX> <LENGTH> <DATA> <ETX> <CRC>
Consider IEEE 802.15.4 Protocol superframe structure
(Image Src: Google)
IEEE 802.15.4 Superframe Structure
In this structure Contention Access Period(CAP) is always followed by Contention Free Period(CFP).
So is there any particular reason behind keeping CAP first and then CFP? Could it be other way around?
Thank you.
It can't really be the other way around because that is what is in the standard. Obviously, you are free to implement your own use of the radio but then I guess it wouldn't be 802.15.4!
The designers of the standard probably had good reason to place the CAP before the CFP (and if you are really interested I imagine it will be documented somewhere in the IEEE meeting minutes etc). My guess is that I think it would have these following benefits:
devices have to wake up their receiver to listen for the beacon frame, and thus if they have any ad-hoc comms to perform (like collecting a pending message or negotiating a connection etc) they can do it straight away and then go to sleep for the rest of the superframe
having the CAP first allows any devices that do not have a GTS to power down their radio for as long as possible
having the CAP first provides time for devices to negotiate a GTS before the CFP starts, thus reducing the latency to their first GTS (i.e. it would be possible to hear a beacon, associate, and obtain a GTS prior to the very next CFP)
I'm trying to pick up some serial comms for a new job I am starting. I have done some reading which has helped a lot however, a lot of the reading tells you about the specification of serial comms and what everything is, but not when is best to use particular options.
My searches for this information so far only seem to pull in the spec; perhaps as a novice I am searching for the wrong terms.
My questions then!
Baud Rate - I have read this is signal changes per second and is often mislabelled as bits per second. Is this essentially bits per second including the frame data if asynchronous, and actually bits per second if synchronous?
Parity - Even/Odd.. Is there any difference at all between the two? I'm thinking in terms of efficiency or similar. Does this only still exist for compatabilities sake?
Stop Bits - I have read so far you can have 1 or 2 stop bits. In C# there seems to be an option for 1.5 too. I can't find anything on why you would want/need more than 1.
If anyone can advise on these points, or point me to some recommended reading material I would be very grateful.
Thanks for reading.
edit: typo
You very rarely have a choice, you must make it compatible with the settings that the device uses. If you don't know then you need to look in a manual or pick up a phone. Do keep in mind that it is increasing very rare to work with a real serial port device, one that uses an UART. Most commonly you actually talk to an emulated serial port, implemented by a USB or Bluetooth device driver. The settings you use don't matter in such a case since the actual signaling is implemented by the underlying bus.
If you can configure the device then basic guidelines are:
Baudrate is directly related to the length of the cable and the amount of electrical interference that's present. You have to go slower when you get bit errors. The RS-232 spec only allows for a maximum of 50 ft at 9600 baud.
Parity ought to be used when you don't use an error-correcting protocol. It does not matter whether you pick Odd or Even. Odd people pick odd, it's their prerogative.
Stopbits is usually 1. Picking 1.5 or 2 help a bit to relieve pressure on a device whose interrupt response times are poor, detected by data loss.
Databits is almost always 8, sometimes 7 if the device only handles ASCII codes.
Handshaking is an important setting that never stop causing trouble since many programmers just overlook it. Modern computers are almost always fast enough to not need it but that's not necessarily true for devices. The most basic stay-out-of-trouble configuration is to turn DTR on when you open the port and to tell the device driver to take care of RTS/CTS handshaking. Xon/Xoff handshaking is sometimes used, depends on the device.
A good 90% of the battle is won by implementing solid error checking. It is almost always skimped on, bad idea. Very important for serial port devices since they have no error correcting capabilities themselves and very weak error detection. Always make sure that you can detect and properly report overrun, parity and framing errors. And test them by getting the settings intentionally wrong.
I'm currently designing a sensor network that will have small ATtiny85 probes that each have a temperature sensor, a barometer, and a humidity sensor. I think I will use these (http://goo.gl/TqaDjl) to communicate as they are low cost and don't need much range. Im not sure though how I will get the probes to communicate with the main control, as the transmitter transmits digitally and I will have +20 probes that all need to send data without signals overlapping or getting messed up every minute. I think the easiest way would be to time the probes so that they don't overlap in transmission but I'm not sure.
Questions:
-Is using RF the cheapest and best option for this system?
-How can I prevent communication overlapping?
-What is the easiest way to send data digitally from an arduino (or ATtiny85)?
I guess I'm late to the party, but I'll offer some insight into collision control with a ton of chattering transmitters on one link, a la 802.11. This is somewhat packetized.
If two transmitters try to transmit at the same time, you're bound to get a mangled mess of rotten bacon at the receivers.
A simplified version of WiFi-style collisions would be good. Basically, it uses preambles that can be detected, and for longer transmissions that have a higher chance of conflicting, it can use shorter request/clear to send packets.
While this is likely overkill, I'd go for preambles. Start by transmitting a steady stream of something recognizable, like in hex, 555533330f0f00ff which is basically alternating 1s and 0s but with changing frequency(0101, then 0011, then 00001111, and so on), a readily recognizable pattern that is unlikely to be given off by stray radiation or noise.
This pattern could undergo a shift so there's a finite set of other preambles that should be bitwise-shifted relative to the original.
If a transmitter detects this preamble, it should STOP and wait. If you limit all packets to a certain temporal length, collisions should not occur if you wait sufficient time between packets. If during the time of one packet, a preamble is heard, then your station should wait for the full length of the transmission(listening to its length and other header fields so it knows how long to wait). Once the packet is done, your station can transmit its preamble.
This is where the WiFi resemblance stops and simpler protocols take over.
Note that if 2 stations are waiting on a packet they can start their preambles almost simultaneously. To resolve this, each station should have a different zero bit flipped in its preamble. If it detects a 1 for that bit, it sees that there's another station preambling, and should back off.
Each station should wait a certain delay(up to you) after each packet so other stations can start their transmissions.
A few sketches of the communication patterns show that this is sufficient for your needs.
Now if it's a master-slave-style system as long as you only have one network it should be easier since there should only be one outstanding request that would involve a slave transmitting.
Those will be by far the cheapest method. As for the best method, there are a variety of choices much better, but more expensive. A network of Xbee modules comes to mind, but those are much more expensive than $1.25 a pair.
Using the RF modules is very do-able however. To prevent communication overlapping, put a RF transmitter and receiver on each sensor node and the main hub. The main hub can send "hey sensor1 give me your data", which gets broadcasted to all of the sensors. However, only sensor1 will realize "hey I am sensor 1, here is my data" which the hub will listen for. Then, the hub will go on and say "hey sensor2 send me your data" and so on and so forth.
I think your original approach may be best. The approach of putting a Tx and Rx on every device may be affordable, but I question if it will work. With 20 devices transmitting on the same frequency, which one will the receiver "hear". Most important, how will a device receive any remote transmitter's signal when its own transmitter is very close? Keep in mind: these are AM radios and will "send" a carrier even if not sending any data. Get a small number of transmitters before trying to go full scale.
To avoid the problem of receiving the one active transmitter among the soup of inactive transmitters, you want only 1 transmitter powered at 1 time. You would control Vcc to one transmitter, turn it on, send the burst of data, and then power it off.
-How can I prevent communication overlapping?
You can't -- you have to accept that there will be occasional overlaps. Add a CRC to the transmitted data so that the receiver can detect garbage.
The timing of the multiple transmitters is surely a project in itself. You surely don't want to run them all at the same transmission period. They may not collide at the beginning, but when two devices did drift together and start colliding, they would stay together and collide for a long time, until the clocks drifted apart.
I would start with something simple. For example with three devices, run the transmissions at 2000 ms, 2200 ms, 2400 ms period (use EEPROM to configure). That way, if a pair happens to collide at one data point, then next transmissions that pair will be 200 ms apart.
I'm currently developing a small application for monitoring the power / current our solar collector is generating.
The array is connected to 3 inverters. Every inverter has a RS232 interface, transmitting one Line of information(its current status) every 10 seconds.
Since I want to do the monitoring using a device only having one serial port, I need to come up with a way to be able to read the data from all of the inverters in parallel.
I don't need to send anything to one of the inverters!
Is it possible to just connect 3 RS232 wires in parallel to one serial port? Collisions will be pretty unlikely since every inverter is transmitting only 64Byte / 10seconds ending with a newline, so I could check for variable line lengths to detect collisions.
I'm sort of chuckling at doomsday and wacky answers that so often pop up on stackoverflow...
But anyway, in years gone buy I have used paralleled RS-232 transmit lines using diodes and it can work fine for situations where collisions are unlikely. In one particular application I used this technique there were two input terminals where a user could key in simple commands to control the system (a specialized security system) and it was very unlikely that two people would be trying to control it at the same time from the two different terminals. Amazingly enough there are no problems with voltage levels with most RS-232A receivers I tested at the time and they tolerated the signal characteristics (no negative voltage) that result from the simple use of the diodes in series with the TXD signals. However, if I had to do this again I would likely add a simple pull-down resister and capacitor to ground with a diode between RXD and the cap in a sort of charge pump configuration or a pull-down to negative going handshake signal to ensure the "OR'd" input signal goes truly negative since the RS-232 spec defines +3 to -3v as invalid.
In any case, I would recommend not using this technique except in very specific, limited, and non-mission critical cases and would not use it in the case where you have multiple devices sending information at a programmed interval as in the case of the OP or where there is a software handshake.
In can be a simple solution to the problem of not enough serial input ports but only in a very limited set of environments.
No, you should NOT connect 3 serial output port in parallel. If you do that you are probably going to broke the RS232 output circuitry of your inverters.
You have 3 RS232 outputs, so you need 3 RS232 input, then you can manage these 3 input the way you like: maybe you can buffer the data from each input, and reoutput the data on a single RS232 output, to be connected to your monitoring device.... but you should add some code in the data flow to differentiate the data coming from the 3 inverters.
Maybe you can use some kind of IC that do the job for you, I'm not sure, but maybe that some IC that multiplex multiple RS232 input on a single RS232 output already exist.
Try this search: rs232 port input multiplexer on Google
Or, if the monitoring device is a Window computer, you can use 3 serial-to-usb converter: that will create 3 virtual COM port on your computer and you can read data from them with any software.
Update
About the hypothesis of securing the output circuitry using diods to block reentering current, I don't think it's going to work...
Many year have passed by since last time I've used an RS232 link at low level (so maybe I'm wrong) but I think that there is some kind of handshake going on between RS232 input and output port (speed to use, parity, stop bit...).
Each RS232 port have inputs and outputs signal, both for data and for transmission control, so your multiple RS232 outputs does have some input signals, and your single RS232 input does have some outputs.
This mean that your input monitoring RS323 port is going to try to make a handshake with 3 RS323 ports at the same time... and the 3 RS232 ports are probably going to respond at the same time... so I think it's not going to work.
Other than that if you place diodes on your output, you are going to loose 0.7v, I don't remember the tolerance on signal level of RS232, but maybe that 0.7v can be relevant.