Bay Technical Associates DS74 - Manual (Page 9)

DS74 Serial I/O Module
9
Word Size
The word size is the measurement of the actual data bits in a transmission. Which setting you choose
depends on what information you are transferring. For example, standard ASCII has values from 0 to 127 (7
bits). Extended ASCII uses 0 to 255 (8 bits). If the data being transferred is simple text (standard ASCII),
sending 7 bits of data per packet is sufficient for communication. A packet refers to a single byte transfer,
including start/stop bits, data bits, and parity.
Select 3), to get the Word Size, Default is 8 the Host module responds:
Type a number to select the Word Size and press <cr>.
Stop Bits
The Stop Bits are used to signal the end of communication for a single packet. Since the data is clocked
across the lines and each device has its own clock, it is possible for the two devices to become slightly out of
sync. Therefore, the stop bits not only indicate the end of transmission but also give the computers some
room for error in the clock speeds. The more bits that are used for stop bits, the greater the lenience in
synchronizing the different clocks, but the slower the data transmission rate.
Select 4), to get the Stop Bits, Default is 1 the Host module responds:
Type a number to select the Stop Bits option and press <cr>.
Parity
Parity is a simple form of error checking used in serial communication. For even and odd parity, the serial
port will set the parity bit (the last bit after the data bits) to a value to ensure that the transmission has an
even or odd number of logic high bits. For example, if the data was 011, then for even parity, the parity bit
would be 0 to keep the number of logic high bits even. If the parity was odd, then the parity bit would be 1,
resulting in 3 logic high bits. This allows the receiving device to know the state of a bit so as to enable the
device to determine if noise is corrupting the data or if the transmitting and receiving devices' clocks are out
of sync.
With no parity selected (or defaulted), it's assumed that there are other forms of checking that will detect any
errors in transmission. No parity also usually means that the parity bit can be used for data, speeding up
transmission. In modem-to-modem communication, the type of parity is coordinated by the sending and
receiving modems before the transmission takes place.
Select stop bits:
1 For 1
2 For 1.5
3 For 2
Enter Request :
Select word size:
1 For 5
2 For 6
3 For 7
4 For 8
Enter Request :