What is the difference between using mark/space parity and parity none?

Question 1

So there are five possibilities, not three: no parity, mark, space, odd and even. With no parity the extra bit is just omitted in the frame, often selected when the protocol is already checking for errors with a checksum or CRC or data corruption is not deemed likely or critical.

Nobody ever selects mark or space, that's just wasting bandwidth. Modulo some odd standard, like 9-bit data protocols that hardware vendors like to force you to buy their hardware since you have no real shot at reprogramming the UART on the fly without writing a driver.

Question 2

There is a very simple and very useful reason to have mark or space parity that appears to be left out here: node address flagging.

Very low-power and/or small embedded systems sometimes utilize an industrial serial bus like RS485 or RS422. Perhaps many very tiny processors may be attached to the same bus.

These tiny devices don't want to waste power or processing time looking at every single character that comes in over the serial port. Most of the time, it's not something they're interested in.

So, you design a bus protocol that uses for example maybe 9 bits... 8 data bits and a mark/space parity bit. Each data packet contains exactly one byte or word (the node address) with the mark parity bit set. Everything else is space parity. Then, these tiny devices can simply wait around for a parity error interrupt. Once it get's the interrupt, it checks that byte. Is that my address? No, go back to sleep.

It's a very power-efficient system... and only 10% wasteful on bandwidth. In many environments, that's a very good trade-off.

So... if you've then got a PC-class system trying to TALK to these tiny devices, you need to be able to set/clear that parity bit. So, you set MARK parity when you transmit the node addresses, and SPACE parity everywhere else.

Question 3

Setting mark or space parity is useful if you're generating data to send to hardware that requires a parity bit (perhaps because it has a hard coded word length built into the electronics) but doesn't care what its value is.

Question 4

Very clear and helpful answers and remarks. For those who find the concept perverse, relax; the term is a problem of semantics rather than information theory or engineering, the difficulty arising from the use of the word "parity".

"Mark" and "space" bits are not parity bits in those applications, and the term arises from the fact that they occupy the bit position in which a parity bit might be expected in other contexts. In reality they have nothing to do with parity, but are used for any relevant purpose where a constant bit value is needed, such as to mark the start of a byte or other signal, or as a delay,or to indicate the status of a signal as being data or address or the like.

Accordingly they sometimes are more logically called "stick (parity) bits", being stuck in "on" or "off" state. Sometimes they really are "don't cares".

Question 5

RS485 requires 9 bits transmission, as described above. RS485 is widely used in industrial applications, whatever the controlled device 'size' (for instance there are many air conditioners or refrigerators offering a RS485 interface, not really 'tiny' things). RS485 allows up to 10Mbs throughput or distances up to 4000 feet. Using the parity bit to distinguish address/data bytes eases hardware implementation, each node of the network can have their own hardware to generate interrupts only if an address byte on the wire matches the node's address.