Using two serial ports as a bi-directional pipe

Don French

I want to use a BS2 with two serial connections and pass all bytes coming in either port out the other port. I tried the using the timeout feature of SERIN, as in the following:

Baud CON 84
Timeout CON 2

DO
SERIN RX\RTS,Baud, Timeout, NoData1, [noparse][[/noparse]dataByte]
SEROUT 16, Baud, [noparse][[/noparse]dataByte]
NoData1:
SERIN 16, Baud, Timeout, NoData2 [noparse][[/noparse]dataByte]
SEROUT TX\RTS, Baud, [noparse][[/noparse]dataByte]
NoData2:
LOOP

With a timeout value of 1, no characters are ever transmitted. With a timeout value of 2, it does fairly well, only dropping every third or fourth character. With a timeout of 3 and higher it drops about 3 out of every 4 characters.

Is there a way to do this that drops no characters? FWIW, I am using a HomeWork board and using the programming port for one port and a XBee RF modem for the other port.

allanlane5

One problem you have is that the BS2 has no serial buffer. If the code is not sitting in a SERIN statement, any bytes coming in will be ignored.

Another problem you have is that it looks like you're trying to recieve (and pass on) one byte at a time. Each byte takes 1 mSec (at 9600 baud) to be recieved, but the SERIN statement takes like 500 uSec to execute. So there's a LOT of time there the BS2 is not 'listenting'.

Don French

Any suggestions on how to solve these problems given an asynchronous bi-directional data stream?

Mike Green

The only way would be to use a 3rd party serial input buffer in each direction. These are usually PICs preprogrammed with a small program to receive a serial stream and buffer it, then retransmit it with flow control to a Stamp or equivalent. They're typically an 8-pin part that either works without any external components or may require an external resonator. I've not had any personal experience with them, but I've seen 2 different sources on websearches. One example is: www.proteanlogic.com/product_periph_rs232.shtml.

Chris Savage

I would recommend using an SX for this task. There have been a few examples of redirecting multiple serial inputs (even of different baud rates) to a single serial output using the SX. It has more than enough horsepower to do the job. Take care.

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Chris Savage
Parallax Tech Support

Don French

Erk! I was hoping someone would know how to do it with what I have here. Oh, well! Thanks to all who replied!

Chris Savage

Don,

Since the BASIC Stamp cannot monitor and receive Serial streams from more than one port at a time I don’t see how it could be done with a BASIC Stamp unless you have a large external FIFO buffer, but again, this wouldn’t be using what you have.

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Chris Savage
Parallax Tech Support

allanlane5

You could also try one of these:

http://www.rhombus-tek.com/co-processors.html

The "Simple Multi-Tasking" processor has a recieve-only UART buffer built in.
You get info from it using SHIFTIN/SHIFTOUT.

Don French

Allan,

That's pretty cool and not very expensive either! Thanks!

-- Don