Remote TOUCH16 sensor, mitigating lead length issues

[Eclectica]

Hi there,

I am setting up a remote TOUCH16 based sensor which houses a PICAXE-08M2 which transmits 4800bps readings when polled by a 28X2 base unit via a 'request data' signal line.

I have read that the TOUCH16 command is affected by long leads on input pins and the presence of the serial programming cable, so:

I plan to optoisolate the 'request data' input signal line at the 08M2 (although common is still used), in order not to connect a long length of cable directly to an input pin.

On the serial output I plan to use a TLC271 as an output buffer so as not to hang a long cable directly from the SEROUT pin for the same reason.

Cable is individually screened twin twisted pair, signal line run twisted with 5V, buffered serial out run twisted with 0v, length 5-10M.

Since there are so many here doing great and wonderful things with the PICAXE chips, I was wondering if anyone had any experience doing something similar?

I can post a schematic, but I'm not sure it would really help since all I am just trying to establish at this stage, are ways of getting round 5-10M cable lengths causing interfering with TOUCH16 command output.

Thanks in advance!

Cheers,
Eclectica.

 

[hippy]

With the touch sensor close to the 08M2 and long serial lead to a 28X2 there may not be any particular problems with length of cable. I would suggest prototyping it before adding additional complexity that may not be necessary.

The issues with programming cables are negative voltages and above power supply signals which can be the case for RS232 cables. For AXE027 cables there should be no such problems nor problems with a purely data link rather than a download programming link.

 

[Eclectica]

Thanks hippy,

Looks like prototype is the way to go.

As a matter of interest, on shortish cable runs, have you always gone for designed RS232 line driver/receiver pairs, or gone down the output buffer route using an opamp, - or even just run the serout pin directly?

Thanks again.
Eclectica

 

[hippy]

For short runs I have usually just gone pin to pin.

 

[Eclectica]

Interesting - definitely a simplified approach is looking promising then.

Much appreciated - thanks.