3 replies to this topic

[Chahu]

I'm using a NP2 and need to communicate to a power meter. This device only supports 300 baud, 7 data bit, even parity an 1 stop bit.

I made some tests and I guess that the lowest baud rate supported from of NP2 is 1200. Is there any trick to run one NP2s serial port with only 300 baud?

 

If not, is a software solution, like SerialSoft for Arduino, possible? Or has anyone experience with I2C/SPI to UART devices, and are there breakout boards or shields available?

 

 

[hanzibal]

If the meter unidirectional and "read only": UART signals are quite simple active low pulses, look it up on wikipedia and you'll see how to interpret the signal. At 300 baud, it should possible to read using an interrupt enabled GPIO. You would then analyze the time difference in between the slopes. Also, It just might be possible to read it at 1200 and then convert the data afterwards (you would typically receive each bit four times). Start och stop bits might present problems though. If the meter is bidirectional, I think you could use one of these: https://www.sparkfun.com/products/9981

[Chahu]

@ hanzibal

Thanks for your response.

The communication to the meter is unfortunately bidirectional. I have to send a initial sequence to the meter to start data output. So checking the received data would be possible with interrupt and timing analyses, but sending the initial sequence reliably is not possible. The NP2 isn't a real time system.

I think if I would use 1200 baud for receiving instead of 300 I will get framing errors, because the UART checks the parity bit and the correct timing and occurrence of the stop bits. Checking parity bit can be bypassed by using 8 data bit and no parity bit.

 

First I will try the above mentioned module.

After some tests I have to decide to use this module further on, or to design a new shield with either the NXP [color=rgb(51,51,51);font-family:Arial, 'MS Sans Serif', Geneva, sans-serif;font-size:13px;]SC16IS740[/color] or Maxim Max3100.

[hanzibal]

Yes, a soft UART would work for receiving since interrupts are correctly time stamped but sending is trickier.

 

Still, if the meter doesn't really use full duplex (won't receive and respond in parallel) you might get away with sending too - 300 baud means >1 ms between pin toggles and that is a pretty long time even for an NP2.