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communicating with a microcontroller

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Member level 3
Feb 15, 2006
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hi all.
iam planning a project in which i need to communicate with a microcontroller(i will use 8051) which is nearly 1km away.
in the other end i wiil use some DTMF my microcontroller sholud read dtmf signals and control.
guide me.

There are two options:

- twisted pair(s) and RS-485 (RS-422)
- radio-modems (wireless)

Which is your preferred way?


Depending on your budget, you can choose professional radio modems, and there are plenty of types offered on the NET, but you can consider much cheaper option using small 434MHz modules from, for example, Oatley Electronics ..
Have a look at RX434 and TX434 modules at:
**broken link removed**
Of course, for 1km distance you will need "line-of-sight" and Yagi antennas ..


I'm not sure those cheap little modules are going to work well at 1000M without some amazing antennas, proper matching and the wind at your back. Definitely a challenging first rf project, in my opinion. I know they are spec'd for 1KM but the guys at sparkfun don't recommend them for more than about 100M (100 yrds, actually). Depending on the laws in India, it might be possible to run them with a higher voltage. I believe the 12V spec limit is due to the US comm laws about limiting to 10 mW. might be worth a try...

Wish I had a better answer. sorry.

hi ian &philba.
now iam getting what u r saying.yes i should go for either RS486 or the radio modems.
but plz explain how to communicate usig RS-486 .

pandit_vlsi said:
plz explain how to communicate usig RS-486 .

Ben shares from his experience with RS-485 by introducing BenBus protocol.

**broken link removed**

i was just searchin for somethin else ... but thought to just drop by n help....
ive used 8051 (AT89c51) with plain serial comm of the external RSx32 or RSx86 connected....over a distance of 4 kms....simple twisted pair....
the only external support i used was CD4016 a quad bilateral a shaper/amplifier IC to cater for line interferrences....
A simple...reliable...and cheap solution...

hi xischaune.
really iam not getting how ur using these twisted pairs to communicate...
plz elaborate........

xischaune said:
connected....over a distance of 4 kms....simple twisted pair....

You got be kidding.

As throwaway18 said :
Data rate, maximum latency?

xischaune said:

I'm in doubt if it's reliable.

Cheap ? Hmmm....

You deserved to be hired by those circuit manufacturer companies involved in RS485 market.

well pandit...
i used serial async UART mode of the microcontroller...
no RSs connected....
the only amplfier/shaper IC used was CD4016B with both inputs same as the incoming signal...
CD4016 was used at both transmitting and receiving end...
so the inverting effect of 4016 was nullified...
you can use a twisted pair or a parallel wire usually used for telephone ur home...
i hope this will be sufficient...

xischaune said:
so the inverting effect of 4016 was nullified...

What are you talking about ? What inverting effect ?

CD4016 it's only a bilateral switch used indeed for transmission or multiplexing of analog or digital signal.
If has been used for switching 20v digital signal on your twisted wires that doesn't mean it's a "amplifier/shaper".
If you think that an extended common-mode range of the driver will help you to overcome the cable resistance and distributed capacitance, well it's your choice for a "A simple...reliable...and cheap solution... "
If CD4016 was used only to switch between RX and TX (the other wire of twisted cable is the gnd) then 4 km it's only in your dream.
If you believe that differential transmission line was developed just for fun then indeed as you said:
"I hope this will be sufficient."

Dixit !

xischaune said:
you can use a twisted pair

Do you know why is used twisted ?

For long distances current loop could be solution , of course very low data transfer rate , or use POTS (does telephone line to be emulated ?) and non POTS modems.
If not retransmission points (to compensate transmission loss) have to be installed

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