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t;>Used up all the good resources of a PIC16F886 to make a high >>resolution ADC (well, soon to be built, but gone through all >>the required PIC resource allocation), now I need to talk to >>another 16F886 (28pin) or 16F887 (40pin) PIC for RTC (32kHz >>watch xtal) and system control. >> >>The ADC chip already taken the EUSART for serial comms to PC >>option. That leaves the other comms module: >> >>" >>Master Synchronous Serial Port (MSSP) Module >>supporting 3-wire SPI (all 4 modes) and I2C >>Master and Slave Modes with I2C Address Mask >>" -- 16F886 datasheet >> >>for talking to other chips and PICs. >> >>I'm after suggestions for which master / slave inter-chip >>protocol is easy, reliable for local (within a couple feet) >>comms. Gotchas, pointers? >> >>I'm programming in assembler with the MPLAB environment at this >>time. >> >>Thanks in anticipation, >>Grant. > > > If you dream up some protocol, you can put everything on the same RS232 > (OR the outputs). > This is sort of what I do in my PIC LED color controller. > > For example PIC1 listens to commands starting with ctrlA, > PIC 2 to commands starting with ctrB, etc. > And each can reply with a sequence like ctrlX<ID><data> > There are a zillion ways. > 115200 Bd is fast enough. > The alternative is to have a software RS232 output, as I am doing in a PIC project I am doing now. > You can still use interrupt on input too. > I have even done I2C in a PIC in software using an interrupt pin, > emulating a PCF8574 I/O expander (and cheaper then one :-) ). MODBUS/RTU on a balanced twisted pair( 485 ) works well, the MODBUS supports 31 devices.. A well known protocol, why reinvent the wheel?

Re: Which multi-PIC mid-range comms protocol? - Tauno Voipio - 2010-08-12 03:38:00

Jamie wrote:
>> emulating a PCF8574 I/O expander (and cheaper then one :-) ).
> MODBUS/RTU on a balanced twisted pair( 485 ) works well, the MODBUS 
> supports 31 devices.. A well known protocol, why reinvent the wheel?

Implementing MODBUS/RTU packet timing to the specification
is a PITA. It demands tight character-to-character timing
and measurement of interpacket timings. This prevents using
the built-in FIFO's in the serial chips, and the timing is
practically impossible to implement on a PC due to the
unpredictable interrupt latencies introduced by operating
system.

Been there - bit badly by that.

-- 

Tauno Voipio
tauno voipio (at) iki fi

Re: Which multi-PIC mid-range comms protocol? - Grant - 2010-08-12 06:12:00

On Thu, 12 Aug 2010 10:38:24 +0300, Tauno Voipio <t...@notused.fi.invalid> wrote:

>Jamie wrote:
>>> emulating a PCF8574 I/O expander (and cheaper then one :-) ).
>> MODBUS/RTU on a balanced twisted pair( 485 ) works well, the MODBUS 
>> supports 31 devices.. A well known protocol, why reinvent the wheel?
>
>Implementing MODBUS/RTU packet timing to the specification
>is a PITA. It demands tight character-to-character timing
>and measurement of interpacket timings. This prevents using
>the built-in FIFO's in the serial chips, and the timing is
>practically impossible to implement on a PC due to the
>unpredictable interrupt latencies introduced by operating
>system.
>
>Been there - bit badly by that.

Not where I'm heading, more likely a tiny serial <-> usb gateway 
to PC for control and data logging, and separately SPI or IIC to 
other chips, I don't need rs485 to talk a foot or two between 
chips for this project set.

Grant.

Re: Which multi-PIC mid-range comms protocol? - Jamie - 2010-08-12 19:18:00

Tauno Voipio wrote:
> Jamie wrote:
> 
>>> emulating a PCF8574 I/O expander (and cheaper then one :-) ).
>>
>> MODBUS/RTU on a balanced twisted pair( 485 ) works well, the MODBUS 
>> supports 31 devices.. A well known protocol, why reinvent the wheel?
> 
> 
> Implementing MODBUS/RTU packet timing to the specification
> is a PITA. It demands tight character-to-character timing
> and measurement of interpacket timings. This prevents using
> the built-in FIFO's in the serial chips, and the timing is
> practically impossible to implement on a PC due to the
> unpredictable interrupt latencies introduced by operating
> system.
> 
> Been there - bit badly by that.
> 
Oh really, we've never had any issues implementing that in small
devices. I really don't see the problem? The master is the one
that sends data to a specific device and also makes request for
the device to return data. The slave device does not tx nothing
until its asked too. And that, obviously doesn't happen until
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