Alexander:

Thanks for the response. It prompted me to do a lot of reading.

Showing my age, parity was always the MSB of an 8-bit byte. There also appears to be an unfortunate reuse of the term "parity". In some implementations of RS-485 systems, the 9th "parity" bit is used to key the end nodes that the current byte is an address(see MAX 3140 datasheet). This allows the end node to quickly determine if the message is for it without having to decode the packet.

This 9th bit is generated internally by the UART based on the value of the parity bit. So, which parity bit is the Perl modules dealing with? And do the modules allow this definition to be changed at will to drive the address function out onto the RS-485 bus?

Interesting topic.

James

This 9th bit is generated internally by the UART based on the value of the parity bit. So, which parity bit is the Perl modules dealing with?

Perl does not do any parity, neither does any other language. The parity bit is generated and checked by the UART hardware. All that the OS and any language on top sees is a parity error if the check fails.

And do the modules allow this definition to be changed at will to drive the address function out onto the RS-485 bus?

That's an application of the mark and space "parity" bits. You could switch the serial format between "mark parity" and "space parity" to set or clear the parity bit from the application.

Of course it is possible to generate and verify the parity bit in the application, that's why some embedded U(S)ARTs allow 9 bit formats (8 bit plus application-specific use of the parity bit). On a generic PC-style USART, the data bits are limited to 8 bit, so if you want to use an application-specific parity bit, you are limited to 4 to 7 data bits.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

The MAX3140 datasheet description of 9bit mode:

The MAX3140 supports a common multidrop communication  technique  refe
+rred  to  as  9-bit  mode.  In  this mode,  the  parity  bit  is  set
+  to  indicate  a  message  that  contains  a  header  with  a  desti
+nation  address.  Set  the MAX3140’s  parity  mask  to  generate  int
+errupts  for  this condition.  Operating  a  network  in  this  mode 
+ reduces the  processing  overhead  of  all  nodes  by  enabling  the
+ slave  controllers  to  ignore  most  message  traffic.  This reliev
+es  the  remote  processor  to  handle  more  useful tasks.In  9-bit 
+ mode,  the  MAX3140  is  set  up  with  eight  bits plus  parity.  T
+he  parity  bit  in  all  normal  messages  is clear,  but  is  set  
+in  an  address-type  message.  The MAX3140’s parity-interrupt mask g
+enerates an interrupton  high  parity  when  enabled.  When  the  mas
+ter  sends an address message with the parity bit set, all MAX3140  n
+odes  issue  an  interrupt.  All  nodes  then retrieve the received b
+yte to compare to their assigned address.  Once  addressed,  the  nod
+e  continues  to process each received byte. If the node was not addr
+essed,  it  ignores  all  message  traffic  until  a  new address is 
+sent out by the master.The parity/9th-bit interrupt is controlled onl
+y by the data in the receive register and is not affected by data in 
+the FIFO,  so  the  most  effective  use  of  the  parity/9th-bitinte
+rrupt is with FIFO disabled. With the FIFO disabled, received  non-ad
+dress  words  are  ignored  and  not  even read from the UART.
[download]

The parity bit that this is referring to is a bit in an on-chip transmit control register. This register has to be modified to cause the chip generated parity bit to be set to indicate an address byte. And then be modified again to turn off address-mode.

My guess is the Device::SerialPort module doesn't use it's parity argument for this purpose.

James

There's never enough time to do it right, but always enough time to do it over...