Serial Comms with SX48 1 wire or two

SailerMan

I have serial comms between a prop and an SX48 so I'm not sure which forum to post this in.

Basically, I have a 2 wire communication working perfectly... I'm using FullDuplexSerial on the prop and standard Serin/Serout on the sx48 using sx/b.

Any how I wanted to convert this to a single wire. So I just chose the same pin for Serial Input and Output on both Micros... Now I can send from the Prop and it recieves well on the sx48, but I send from the SX to the prop... Any Ideas?

Eric

Mike Green

FullDuplexSerial is just that ... It's a full duplex routine. The output pin is always enabled for transmitting and the input pin is always enabled for receiving. During initialization, the way the Propeller works, if you use the same pin number, the output mode will take precedence.

The FullDuplexSerial routines have a mode parameter. If you set the mode to be %1100, this will set open-drain mode and the transmit routine will throw away a received character for each one transmitted (since the receiver is always active it will receive all transmitted characters). Open drain mode will set the I/O to input instead of driving the I/O pin high. It will drive the I/O pin low, but you'll need a pull-up resistor to provide the high logic level. The receive routine will be able to receive between transmitted characters which is what you want.

Try just changing the mode parameter and adding the pull-up if you don't already have it.

SailerMan

OK thanks Mike I will try this when I get home later... One question... Is there anything I need to change on the SX Side?

I'm hoping not as I want the SX (Slave) side to be independant of the (Master).

Eric

Mike Green

I'm not sure what you're doing on the SX side. The main issue is: How do the Propeller and SX agree about who transmits and who receives? There are all kinds of ways to do this ... anything from a strict master/slave relationship to an Ethernet-like protocol where there are no masters, but there are rules about what to do when two or more devices attempt to transmit at a time.

SailerMan

In Simple terms how do you determine who is attemping to transmit? So rules can be made.

In any event the SX is controlling a motor driver board that I am also designing (Yeah I know there are a ton of them out there) but none that I made... It's a total learning experience for me. [noparse]:)[/noparse] Thanks for you insight.

Regards,
Eric

Post Edited (SailerMan) : 10/4/2007 6:26:12 PM GMT

Mike Green

You make up your own rules and your programs implement the rules ... That's it.

The simplest set of rules is that one side of the conversation always initiates:

SX: Talk to me (are you still there?)
Prop: What do you want to know?
SX: Send me a packet
Prop: Here it is
SX: OK, I got it
....
SX: Talk to me (are you still there?)
Prop: What do you want to know?
SX: Nothing really, just checking
...

SailerMan

I finally got this to work.. Thanks.. I actually had to put a 75 uSec delay on the sx48 side between receiving a command and sending the reply.

I was getting unusual numbers on the propeller... I looked at the scope and the bit pattern looked a little poor and it was being sent a little too early it seems that this small delay has got it working.. I don't know why. Hmm.

The SX was sending %1101_0000 but the Prop was receiving %0110_1000. I'll have to look at this in more detail.


Eric

deSilva

Most likely the SX was not waiting until the stop bit time slot was over. No one does - why should one? But of course you must not start transmitting on the the same line before it's over

Post Edited (deSilva) : 10/5/2007 6:13:03 PM GMT

StefanL38

this is just an additional post for easier finding this thread by special keywords


V24 RS232 bidirectional bi-directional oneline onewire FullDuplexSerialbidirectional bidirectionalfullduplexserial oneiopin for bothdirections
V24bidrectional serialbidirectional FDSbidirectional singlewire singleline V24singlewire V24onewire RS232singlewire RS232onewire

Stefan