8E1 Serial with Basic Stamps

max_m · 2009-12-18 21:47

I'm trying to communicate with an industrial Pressure/Level sensor with serial communication parameters of 8E1 (8 Bits, Even Parity, 1 Stop Bit). I'd like to use a basic stamp to poll and process the data from the sensor but it seems the serout/serin command can not handle 8 bits and even parity. Has anyone been able to make a stamp communicate with 8E1 settings? If so, can you share your solution?

Thanks

Mike Green · 2009-12-18 22:03

Look at the chapter on the SEROUT and SERIN commands in the BASIC Stamp Syntax and Reference Manual or the help files for the Stamp Editor. There are tables for the Baud constants for various settings. One group is for 7 data bits, even parity, and a stop bit. This is what you should use. With very rare exceptions, when datasheets mention 8 bits with even parity, they're including the parity bit in the total of 8 bits. There are old devices that have 8 data bits and a parity bit (to make a total of 9 bits), but this is very non-standard now and most modern equipment cannot talk to this sort of device.

max_m · 2009-12-18 22:45

Right, I've actually already tried that but thanks. I can confirm the sensor expects 11-bit packages. Thats 1 start bit, 8 bits for data, parity bit, and stop bit. The sensor documentation specifies it as such. I can communicate with the sensor using hyperaccess set to 8E1 and a simple hyperaccess script.

Mike Green · 2009-12-18 23:44

The Stamps cannot do 8 bit data plus parity. You'd have to use either the SX or Propeller. They both could handle this easily. The Propeller's serial library routines assume 8 bits of data without parity, but one routine (Simple_Serial) just uses a constant of 8. It's a simple matter to change that to 9 and to supply/check the parity along with the data. The Propeller is fast enough to directly calculate the parity like:

data |= ((data<<1)^(data<<2)^(data<<3)^(data<<4)^(data<<5)^(data<<6)^(data<<7)^(data<<8))&$100

Post Edited (Mike Green) : 12/19/2009 12:31:28 AM GMT

max_m · 2009-12-19 00:16

Thanks. We actually have a Propeller eval board here but I just haven't delved into it. It may be the solution.

Mike Green · 2009-12-19 00:28

The following modifications to Simple_Serial should allow you to receive and transmit 9 bits of data rather than 8 with the 9th bit provided for parity. The transmit routine calculates the parity and includes it. The receive routine ignores it.

PUB rx: rxByte | t
{{ Receive a byte; blocks caller until byte received. }}

  if rxOkay
    dira[noparse][[/noparse]sin]~                                          ' make rx pin an input
    waitpeq(inverted & |< sin, |< sin, 0)               ' wait for start bit
    t := cnt + bitTime >> 1                             ' sync + 1/2 bit
    repeat 9                                                 ' *****
      waitcnt(t += bitTime)                             ' wait for middle of bit
      rxByte := ina[noparse][[/noparse]sin] << 8 | rxByte >> 1             ' sample bit *****
    waitcnt(t + bitTime)                                ' allow for stop bit 
    rxByte ^= inverted & $1FF                            ' adjust for mode *****
    rxByte &= $FF                                       ' strip off received parity *****


PUB tx(txByte) | t
{{ Transmit a byte with even parity; blocks caller until byte transmitted. }}

  txByte |= ((txByte<<1)^(txByte<<2)^(txByte<<3)^(txByte<<4)^(txByte<<5)^(txByte<<6)^(txByte<<7)^(txByte<<8))&$100
  if txOkay
    outa[noparse][[/noparse]sout] := !inverted                             ' set idle state
    dira[noparse][[/noparse]sout]~~                                        ' make tx pin an output        
    txByte := ((txByte | $200) << 2) ^ inverted         ' add stop bit, set mode *****
    t := cnt                                            ' sync
    repeat 11                                           ' start + eight data bits + even parity + stop *****
      waitcnt(t += bitTime)                             ' wait bit time
      outa[noparse][[/noparse]sout] := (txByte >>= 1) & 1                  ' output bit (true mode)  
    
    if sout == sin
      dira[noparse][[/noparse]sout]~                                       ' release to pull-up/pull-down

This is actually off-topic for this BASIC Stamp forum. If you have any questions or comments further on this, please start a new thread in the Propeller forum. Thanks.

Post Edited (Mike Green) : 12/19/2009 12:36:01 AM GMT

8E1 Serial with Basic Stamps

Comments