Serial transmission between two PICAXEs

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Minifig666

Senior Member
I was wondering what circuit I would need to send serial data between two PICAXE chips? I see no mention of it in the manuals. Basically I wish to use an 08M as a sort of shift register connected to an 28X1. Also is it possible to use the Serial in/out pins and the SERTXD/RXD commands. Would I still need the download resistors in place if using the serial I/O pins?
 
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kd5crs

Senior Member
Circuit diagram for picaxe to picaxe serial comms:

Picaxe 1 ------- Picaxe 2

Or in other words, just a wire. Serin on one picaxe, Serout on the other.

I think the answer to the resistor question is yes. The only gotcha I know of is that the 08m cannot use SERRXD.

Brian
 
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westaust55

Moderator
As a reminder, in addition to the one wire mentioned by kd5crs, do not forget to tie the 0V lines for the two systems together if they have different power supplies/batteries.
 
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manuka

Senior Member
Wire!? These darlings have such a high input impedance that they'll transfer data almost by proximity. One of my eternal PICAXE party tricks involves serial transmission by damp string, or once even kids touching hands (with adult "terminals" each end). You can even try SWER (Single Wire Earth Return)
 
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Minifig666

Senior Member
The thing is I'm realy tempted to try this. What do you mean by single wire earth return?As in using the earth rail to send signals. I have had pins pulled high just by touching betwen 5V and an input, licking the two bits hurt a bit though.
 
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westaust55

Moderator
SWER in simple terms means sending power or a signal down a conductor and the return path to complete the loop is through the general mass of earth.

This is frequently and still used for high voltage power supplies in some rural areas so only a single power conductor runs past a series of farms with a small single phase step-down transformer at each farm house.

year ago the early telegraph lines uses a sinmilar technique with a single wire on the poles.
 
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westaust55

Moderator
For any integrated circuit and module, it is good/recommended practice to connect all pins/conenctions with the same designation as part of the circuit an not rely upon the possible presence of some internal/on-board conenction between those pins.
 
hippy

hippy

Ex-Staff (retired)
Minifig666 said:
And also just a quickiy, do you have to connect the V+ to both of the 28X's pins?
Click to expand...
Quick answer : Yes.

Longer answer : You don't have to, just as you don't have to look before crossing a road. Sometimes you can get away with it, other times it can be fatal not to. You may always be lucky, you may not.

Technical answer : When a chip has been designed with two or more +V ( or 0V ) they may be essential for correct and guaranteed operation. Not connecting one or the other may cause currents within the chip through tracks which were not designed to take such current. The resistance of those tracks may cause slight imbalances which affect things internally.
 
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Pauldesign

Senior Member
Code: 
And also just a quickiy, do you have to connect the V+ to both of the 28X's pins?
In general, in my opinion if two or multiple power points provided both are either analog or digital or RF power points but not mix signal power lines, buzzes to zero or less than 10ohms depending on the continuity threshold of multi-meters as they varies (Fluke DMM with RoHs compliance recommended ) and the value of the Vcc or Vdd voltages and the type of digital logic levels that will be used, then they can be connected together or otherwise connected separately with short and thick power lines or wires if possible screen in star a pattern to minimize ground loop and cross talks or signal integrity.

I maybe a bit out of topic but just in case.

Rgds
 
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manuka

Senior Member
SWER, although pondered by Tesla, was first tamed by New Zealander Llloyd Mandeno ~1925 & any Kiwi versed in power electrical work soon gets to learn of it. See => http://en.wikipedia.org/wiki/Single-wire_earth_return

In fact the zooming cost of copper has given it a boost in some countries for mains power distribution, as regular galvanised steel wire can be used. This of course is much stronger than copper,so fewer poles are needed, BUT (as an appealing side effect) the wire is less likely to get stolen!
 
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Minifig666

Senior Member
KK Thanks for that. I'll just have to solder a wire betwen the pins then. Its a shame that you cant have the V+ and ser in pn one side amd V- and ser out on the other like the 08M. It is nice and easy to remember, but i supose you are not responsible for these layouts... untill now (18M2). I think salty water coverd string may actualy be viable for serial coms.
 
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kd5crs

Senior Member
That's so cool.

Hey, I bet that is what is actually happening on the Sistine Chapel ceiling. "Hey Adam, check this out, I'm going to send you some data." :D

Brian
 
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Minifig666

Senior Member
I have tried it without the download circuit (Bad engineer, BAD!) and it seems to work fine. I am using the 20M starter board to program them till I get to school on Monday to print out my board layout.
 
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