900 MHz Wireless Modems

BigFoot

Have any of you had any experience with MaxStreams (Digi) 900 MHz
XTend-PKG modems. ? They advertise 1 watt output @ 125 Kbits/sec.

Russ

DavidM

Hi Bigfoot,

I haven't used the MAXSTREAM Products , But I do use the AEROCOMM ( LAIRD TECHNOLOGIES ) Products

I have used the AC4490-1000M, with great success.
lairdtech.thomasnet.com/viewitems/910mhz-modules/ac4490-transceivers?&forward=1

you can get them from here,

search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=AC4490-1000M-ND

It is 1 watt, I have written a lot of code ( to interface to many of its features )
It will connect directly to the prop at 3.3 volts, you may wish to use a separate 3.3v LDO

regards

Dave M

BigFoot

Dave,

Interesting product and small, what kind of range do you get at 115 Kbits/sec ?

Russ

Erik Friesen

I am using the xtends a bit. They don't push brush well, like any 900mhz. When the rssi falls to -95 or so, the communication is not real reliable, if you are using large packets. I would take any 20 mile claim with a grain of salt. It would take high gain antenna with a completely clear fresnel zone.

One advantage to the xtend is that you can buy a complete product that is ready to set in a customers location, where you will have to package the above in your own solution. However, the cost is much cheaper on the above referenced units. I would like to know about them, if they are reliable.

BigFoot

Erik,

One of the reasons we like XTend is the nicely packaged base station, we really only need to go a mile or so but don't want to fool around with fancy antennas.
We also evaluated there XStream units but they didn't have the range or data rate we need. What kinda range do you get from XTend with the rubber duck @ 115 Kbits ?

I looked at the Laird unit above which is interesting and also evaluated RFM's DNT900 which has some cool features, PWM's, D-A's & I/O but no packaged base station.

http://www.rfm.com/products/fhss.php

Russ

Post Edited (BigFoot) : 4/13/2010 7:33:48 PM GMT

Erik Friesen

We are around 3 miles with high gain antenna's through trees. I think you will very easily reach one mile, at least if you get decent line of sight. The rubber ducky's that come with them are not that great, but they will probably work.

DavidM

Hi Erik,

If your situation is a FIXED, POINT TO POINT, I suggest a DIRECTIONAL ANTENNA ( YAGI Style or similar ) not OMNI.


I use the AEROCOMM in a mobile "adhoc" harsh environment, so we need to use omni-directional antennas, One that we use that has a hi gain is from here..

www.l-com.com/item.aspx?id=22181

I also use the RSSI feature and display a BAR GRAPH on a small lcd, this gives me REALTIME indication of signal strength and reliability.

Another thing, if you choose Aerocomm, you can ask me for some code that might help you out. I can post this here.


Thanks
Dave M

BigFoot

Our situation is fixed to multi point, the master is inside a school cafateria and the remotes are in point of sale terminals outside.
They use the systems for selling food at sports events, hot dogs at football games and stuff like that.

The I-Com unit is a nice antenna, a lot of gain and the price is the same as the smaller ones. I am going to try these with the
XTend units.

Russ

Erik Friesen

One issue I see with the aerocomm units is that they have a 250 byte input buffer. I have just been through a complicated situation where 250 would not be sufficient.

Erik Friesen

These laird units are really tempting to me, but in checking them out further, I find that they require a client server relationship. I am afraid that in a spread out situation, or one where the server goes down, the whole network would be compromised. @davidm - thoughts or solutions?

DavidM

HI Erik,

keep in mind these aerocomm units can operate in POINT TO POINT ( Client - Server ), Point to Multipoint ( both using addressed packets, and broadcast packets)

I wouldn't worry about the input buffer, If you use the standard fullduplexserial object I think from memory it has only a 16byte buffer, I have modified this object to take 128 bytes?, I thought I had a an issue with the buffer in one of my applications, But the "SERVER should really try to read and process any data receive as quickly as possible, The "CLIENTS should not transmit so much data unless the SERVER CAN ACCEPT it either, you should also send small packets and send acknowledgements back before any CLIENT can send more data!

You should have a cog ( continually running) that receives all the data from the buffer , then you can store the messages , say in an array of strings and process them when the server has time, now depending on the message , priority can be given to what kind of data should be processed, remember with the RAM in the eeprom and the PROP CHIP , this should serve as your LARGER BUFFER!


What kind of messages/data are you sending?
How big is the data packet/message?


RE: If the server goes down,


If you work out what messages you are sending, for example are you sending just TRANSACTION HISTORY, or do the clients need "AUTHORISATION" from the SERVER to do their stuff?

If its transaction history , then I would save this at each clients unit, and then send the history back to the server when there is sufficient time & "space in the buffer" to do so.

I guess that the SERVER would be controlling each client when they should transmit, Similar to an RS485 network situation.

please note I have only used these modems in POINT TO POINT, But I do have a BIG Project soon this year that will be complex, .i.e one server controlling MANY clients, This should be fun!


regards

Dave M

Erik Friesen

I am not sure we understand each other here. If you check out the manual closely, on page 14 of the pdf, here is what it says -

The manual said...
The AC4490 utilizes a Server-Client network where all Clients synchronize their hopping to the Server. The Server
transmits a beacon at the beginning of every hop (20 ms). The Client transceivers listen for this beacon and upon
hearing it assert In_Range Low and synchronize their hopping with the Server.
Each network should consist of only one Server and there should never be two

This would make the system reliant on the server, even in point to point, if I understand what it is saying.

Some of the outlying units are transmitting a 300-400 byte string, there is no way to change how they operate.

DavidM

HI Eric,

Firstly can you show me an example of the string your are sending?

Can the string be broken up into smaller messages?, It would be easier to manage, if there is a dropout, you would only have to resend part of the message and not the whole large string again.

You have to ask yourself what is the purpose of the SERVER, does it serve as an AUTHORISATION point?, or a transaction history collector?

Do you have to send the data from the CLIENT's straight away, or can this be saved and then sent later,

I believe you can have more than one server if you assign different CHANNEL NUMBERS and I think you need to utilise the SYNC to channel feature. ( I have ot used it this way YET!)

Thanks

Dave M

Erik Friesen

This is a legacy system. There is no way for me to program these water management boxes at present. They output this info at 9600 without breaks. These are using a mc68hc11 motorola processor plus a 1mb m271001 eeprom. I wish I knew more about them.

DavidM

Ok,

This makes it more difficult!

Are you interfacing to these units with your own choice of modem & a PROPELLER interface? I would do this as I have interfaced an AEROCOMM MODEM with a serial device directly ( no propeller board) and I had issues with scrambled data, I believe it was the buffer in the serial device. I then used a PROPELLER Interface with the modem, All my data corruption's were eliminated, as I could control the "DATAFLOW" much better. Although this will cost a bit more. I would make BOTH the SERVER & CLIENT units using the same CODE & BOARD ASSY, Its much easier to develop this way.


This would be the only way to manage all the data flying around.

You would need to get hold of any info for the UNITS if you can. There must be some kind of specification for the message data?

Regards
Dave M