Long distance comms

Daveb wrote:
> Hi
> 
> I have a requirement to communicate between a slave device and a master
> where the distance between them is approximately 3km. I don't need high
> speed comms, 10-20kbaud is sufficient. Unfortunately I don't have the
> luxury of being able to use repeaters, fibre optics or wireless. If
> RS422 was spec'd beyond 1.2km then that would be ideal.
> 
> It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd
> rather not have the complexity required to implement CAN especially as
> I only need point-to-point comms. I was thinking of using CAN
> transceivers & connecting the Rx/Tx pins to UARTs at either end.
> 
> Does anyone have any experience of such an application or have any
> suggestions ?

I would care less about the specified lenght of RS422,
which is 1500m, than about common mode issues.
With sufficiently low baud rate, the RS422 in
point-to-point mode might do even a bit longer than
specified. Else a closer look at the attenuation of
the cable might be helpful.

Rene
-- 
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Daveb wrote:

> Hi
> 
> I have a requirement to communicate between a slave device and a master
> where the distance between them is approximately 3km. I don't need high
> speed comms, 10-20kbaud is sufficient.

We used to employ both 60ma current loop and also limited distance
modems for this sort of application; why not consider off-the-shelf
modems and cheap FXS and FXO interfaces?

Michael Grigoni
Cybertheque Museum

In article <1144493923.566431.168140@u72g2000cwu.googlegroups.com>, 
dave.bryan@gmail.com says...
> Hi
> 
> I have a requirement to communicate between a slave device and a master
> where the distance between them is approximately 3km. I don't need high
> speed comms, 10-20kbaud is sufficient. Unfortunately I don't have the
> luxury of being able to use repeaters, fibre optics or wireless. If
> RS422 was spec'd beyond 1.2km then that would be ideal.
> 
> It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd
> rather not have the complexity required to implement CAN especially as
> I only need point-to-point comms. I was thinking of using CAN
> transceivers & connecting the Rx/Tx pins to UARTs at either end.
> 
> Does anyone have any experience of such an application or have any
> suggestions ?
> 
Buy a couple of 28 or 56KBaud  POTS modems.  (Do they still make 
those??)

Mark Borgerson

Hi all,

It looks like a current loop is the way to go. Opto isolating might not
be so easy as I have elevated temperature to contend with (105degC). A
standard modem solution is likely to be unworkable due to space
constraints in the slave device.

Thanks again for all the suggestions!
Dave

Daveb wrote:

> Hi
> 
> I have a requirement to communicate between a slave device and a master
> where the distance between them is approximately 3km. I don't need high
> speed comms, 10-20kbaud is sufficient. Unfortunately I don't have the
> luxury of being able to use repeaters, fibre optics or wireless. If
> RS422 was spec'd beyond 1.2km then that would be ideal.
> 
> It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd
> rather not have the complexity required to implement CAN especially as
> I only need point-to-point comms. I was thinking of using CAN
> transceivers & connecting the Rx/Tx pins to UARTs at either end.
> 
> Does anyone have any experience of such an application or have any
> suggestions ?
> 
> Thanks
> Dave
Dave,

TI has some modern "optimized" RS422/485 transceivers
(SN65HVD05,06,07,20,21,22), which have controlled slewrate and are well
suited for long distance applications. I am using the HVD21 in a 1MBaud,
200m multidrop application. Extrapolating from that 6km at 10kbaud shall
work. IMO the TI RS485 chips are as good or better than the CAN approach
(the CAN transceiver advantage is that their slew rate can be "programmed"
by an external resistor), otherwise using CAN transceivers for UART
protocol should readily work, but noise margin is smaller, as their is no
voltage on the line in the recessive state.

Regards

Thomas

Thomas Gallenkamp wrote:

> Daveb wrote:
> 
>> Hi
>> 
>> I have a requirement to communicate between a slave device and a master
>> where the distance between them is approximately 3km. I don't need high
>> speed comms, 10-20kbaud is sufficient. Unfortunately I don't have the
>> luxury of being able to use repeaters, fibre optics or wireless. If
>> RS422 was spec'd beyond 1.2km then that would be ideal.
>> 
>> It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd
>> rather not have the complexity required to implement CAN especially as
>> I only need point-to-point comms. I was thinking of using CAN
>> transceivers & connecting the Rx/Tx pins to UARTs at either end.
>> 
>> Does anyone have any experience of such an application or have any
>> suggestions ?
>> 
>> Thanks
>> Dave
> Dave,
> 
> TI has some modern "optimized" RS422/485 transceivers
> (SN65HVD05,06,07,20,21,22), which have controlled slewrate and are well
> suited for long distance applications. I am using the HVD21 in a 1MBaud,
> 200m multidrop application. Extrapolating from that 6km at 10kbaud shall
> work. IMO the TI RS485 chips are as good or better than the CAN approach
> (the CAN transceiver advantage is that their slew rate can be "programmed"
> by an external resistor), otherwise using CAN transceivers for UART
> protocol should readily work, but noise margin is smaller, as their is no
> voltage on the line in the recessive state.
> 
> Regards
> 
> Thomas
P.S.: The DC resistance of the wire pair is critical. A 24AWG pair has about
1.2kOhm loop resistance at 6km. Differential voltage at the receiver will
be some 300mV, which is still satisfactory. 

Hello Dave,

> 
> It looks like a current loop is the way to go. Opto isolating might not
> be so easy as I have elevated temperature to contend with (105degC). A
> standard modem solution is likely to be unworkable due to space
> constraints in the slave device.
> 

If isolation is a requirement look into toroids. You'll lose the DC 
component but if you use a protocol that only listens to the transitions 
it should work. Hysteresis receivers, for example.

Regards, Joerg

http://www.analogconsultants.com

Thomas

Thanks for the tip. Unfortunately these
devices are 85degC max & I need 105degC+

Dave

Hello Dave,

> 
> Thanks for the tip. Unfortunately these
> devices are 85degC max & I need 105degC+
> 

You could roll your own receivers using mil/automotive grade chips. For 
example there are versions of the LM139 that go to 125degC.

If differential noise immunity is a concern at such low signal levels 
you might want to look into using coax. You could pick whatever 
impedance comes in a rugged and reasonably priced variety.

Regards, Joerg

http://www.analogconsultants.com

Daveb wrote:
> 
> Thanks for the tip. Unfortunately these
> devices are 85degC max & I need 105degC+

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