Bidrectional Serial Link

H

hal8000

Member
This is my first project and although its working, I am having some intermittent problems.

I am using a network of Picaxes to signal Intercom data, over 2 wires, (bus) and Ov.
My test setup is just 2 stations.

Signalling from Station1 to Station 2 works, every time, however from Staion 1 to Staion 2,
signalling does not work every time, about 1 time in 5 is OK, the other 4 attempts fail.

Schematic is here:
http://i.imgur.com/GnPZO.png

Although this is a single wire system, 1 byte sent and receieved in register b0
is sufficient for all my needs, giving 256 codes,or 64 bytes for 4 stations.

All Picaxes on the bus are not using a calling ID, the byte will be decoded per station.

At the moment this is purely test, and on station1 I have used a Common Anode 7 segment display,
if byte 253 is received, then display will change to 8. Eventually, codes will switch relays, and
a second wire will carry purely audio, no data.

This works but intermittantly. Until I can get stable birectional communication, I cannot
proceed further.

I have narrowed down the problem using debug.
Station 2 always sends but when Station 1 fails to receive, no data is received at Station1
(monitored by debug).
I have a feeling this is a timing problem, maybe I have to use calibfreq command.

There are two different programs, samples below:


Station 1 Program below:

Code: 
#picaxe 18M2    ' define pIcaxe
symbol busdata= b1  ' define variable to hold busdata
let busdata = 0
dirsB=%11111111
dirsC=%11111111
pause 500
let pinsB = %11111111 ' blank 7 segment

main:   	'scan input switches
gosub checkbus '  call checkbus here
if  pinC.2 = 1 then let busdata = 224
pause 10
gosub senddata
let pinsB = %11110000   ' if C2 pressed display 3 and set data to 3
pause 100
let pinsB = %11111111	' blank display common anode
endif
if pinC.1 = 1 then  let busdata = 244 ' in B6 pressed display 2 and set data to 2
pause 10
gosub senddata
let pinsB = %01011011	' digit 2
pause 100
let pinsB = %11111111	' blank display common anode
endif
goto main


checkbus:
' Bus is  pin C.7 on 18M2
if pinC.7 = 1 then  'bus is active
do
if pinC.7 = 0 then exit ' wait till bus is clear
loop
serin C.7,N2400_4,b1   ' received byte stored in b1
debug
let pinsB = b1	' display received bus data on 7 segment
pause 250
let pinsB = %11111111	' blank display common anode
endif
if b1 = 253 then 
let pinsB = %00000000 ' display 8 if input 1 received	

endif
return


senddata:
if pinC.7 = 1 then  'bus is active
do
if pinC.7 = 0 then exit ' wait till bus is clear
loop
endif
'high C.7
'pause 100 ' initialise bus
low C.7
pause 20
serout C.7,N2400_4,(b1)
let dirsC = %00000000 ' set port B pins as inputs 
return


Program for Station 2 using OLED

Code: 
#picaxe 18M2    ' define pIcaxe
' c7 serial link    b3 sounder  pin 8     b4  oled       pin 9
symbol busdata= b1 ' define b1 to hold busdata
let busdata = 0	' C.7 =Pin16 = Serial Link
dirsB=%11111111  ' B.3=Pin9 = Sounder
pause 5000	     ' B.4 = Pin9 = OLED
serout B.4,N2400,(254,1) ' clear oled after 5 secs
pause 30


main:
gosub checkbus ' new position
if  pinC.2 = 1 then   let busdata = 253  ' if C2 pressed display 3 and set data to 253
	pause 10
	serout B.4,N2400,(254,1) 'clear oled
	serout B.4,N2400,(254,128) 'line 1
	serout B.4,N2400,("Calling Unit 3")	
	pause 400 '   wait 0.25 second
	serout B.4,N2400,(254,192) ' Line2
	tune B.3, 2,($16,$18,$19)
	gosub senddata
	pause 2000 ' display 2 seconds
	serout B.4,N2400,(254,1) 'clear oled
endif
if pinC.1 = 1 then  let busdata = 67' in B6 pressed display 2 and set data to 2
	pause 10
	serout B.4,N2400,(254,1) 'clear oled
	serout B.4,N2400,(254,128) 'line 1
	serout B.4,N2400,("Calling Unit 4")
	serout B.4,N2400,(254,192) 'line 2
	tune B.3, 2,($16,$18,$19,$1a)
	gosub senddata
	pause 2000 ' display 2 seconds
	serout B.4,N2400,(254,1) 'clear oled
	pause 30
	endif
'gosub checkbus  ' old position
goto main



checkbus:
if pinC.7 = 1 then  'bus is active
do
if pinC.7 = 0 then exit ' wait till bus is clear
loop
serin C.7,N2400_4,b1   ' received byte stored in b1
pause 10 ' let serin settle
debug
serout B.4,N2400,(254,1) 'clear oled
pause 30
serout B.4,N2400,(254,192) 'line 2
serout B.4,N2400,("Incoming Call ")
serout B.4,N2400,(b1)	' display received bus data on OLED line 2
tune B.3, 2,($19,$18,$16)
pause 2000		' wait 2 second
serout B.4,N2400,(254,1) 'clear oled
pause 30
endif
return



senddata:
if pinC.7 = 1 then  'bus is active
do
if pinC.7 = 0 then exit ' wait till bus is clear
loop
endif
'high C.7
'pause 100 ' initialise bus
low C.7
pause 10 ' delay 10ms for serial in to settle
'serout C.7,N2400,(254,1)
serout C.7,N2400_4,(busdata)
debug
let dirsC = %00000000 ' set port  C pins as inputs 
return


The schematic is purely test at the moment, and sending from 1 to 2 works,
verified with "alpha" and "beta" characters on OLED display.
The opposite way also works, but intermittently.

Bus is held low, via a 10k and 1k resistor (at one end only). Data onto the bus goes via
a 1N4148 diode.

Thanks for any help, insight.

If I dont get this working, I will re-design using a 2 wire system, but would be nice to get this
working.
Thanks in advance.
 
M

mrburnette

Senior Member
@hal8000:
What is the current distance from station 1 to 2? What are the environmental state around the unshielded twisted pair? GND at each station referenced to what; AC power GND, AC power Neutral; ?. It MAY be that you are getting AC introduced into the signaling due to ground-loop. The DC impedance on Station 1 is 1K to GND and on Station 2 is ?? Anyway, I suspect that the DC impedance on both stations should be identical. GND reference must be identical, or you will need to consider shielded twisted pair where the shield is only Grounded at one end, not both. With both stations OFF, use a DVM on the AC scale and see if you have any induced signal from local GND to the cable active conductor. If so, your digital signal is simply riding a wave of AC... not good for a signaling bus.

- Ray
 
H

hal8000

Member
Thanks for your quick reply MrBurnette,
Sorry my diagram is not too clear but the impedance to ground is 1k on both stations as the dotted lines are connected.
That is a good point as its 1k then in series with 10k so it may be 11k at each end, I will experiment with this resistor
value.

GND at each station is also electrically connected by the second wire (so ignore the ground symbol) it should be
more accurately labeled as chasis as they are both the same potential.

At the moment the distance is literally about 3 inches, but I've tried with a small 3 metre length of CAT5 cable and
again it works but intermittently.

I have been testing in front of my monitor, its a flat LCD screen, so could be some RFI/EMI, I'll test again tomorrow.

Im may try lowering baud rate to 1200 or so, see if that helps.

Eventually the distance will increase to about 10 metres between stations,
each working off regulated 5Volt supplies, but the 0volt line will be connected
to each other. Cable capacitance will increase possibly to a few hundred pF at 10 metre
so will simulate this on my breadboard with a fixed capacitor, once I get reliable
operation.
 
C

craigcurtin

Senior Member
Can i suggest you do a search on this forum for Kranenbourg - he has designed and implemented a working 1/2 wire bus for multiple picaxe and published both the code and the schematics/explanation - much easier than re-inventing the wheel

Craig
 
M

mrburnette

Senior Member
... perhaps incorporating the physical link would be a better approach and not be burdened with all the software overhead and complexity. Still 10 meters is well in the range of standard distances that any PICAXE user should be able to reach without too much difficulty.

Craigcurtin- an excellent reference


- Ray
 
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