Watlow Series 988 Family - Manual (Page 9)

Introduction to Data Communications, Chapter 1 Data Communications with the Watlow Series 988 Family
1.5
error. The total number of 1s would be even and a violation of the odd-parity rule.
At Watlow, we use odd, even and no parity.
Odd parity sets the parity bit to 0 if there are an odd number of 1s in the first seven
bits.
Even parity sets the parity bit to 0 if there are an even number of 1s in the first
seven bits.
No parity ignores the parity bit.
Start and Stop Bits
A "start" bit informs the receiving device that a character is coming, and a "stop" bit
tells it that one is complete. The start bit is always a 0. The stop bit is always a 1.
We've added the start and stop bits to the transmitted "W" example.
The human speaking equivalent of these bits could be a clearing of the throat to
get someone's attention (start bit); and a pause at the end of a phrase (stop bit).
Both help the listener understand the message.
7-bit character
1 2 3 4 5 6 7 8
1
0
idle line
start bit
stop bit
odd parity bit
Figure 1.5 - ASCII
upper case "W" with
start and stop bits.
Baud Rate
The baud rate refers to the speed of data transmission. When a change in signal
represents one data bit, baud rate is equal to bits per second (bps). Our rates on
the 988 Family of controllers are 300, 600, 1200, 2400, 4800 and 9600 baud.
Computer Languages
Computer languages are simply sets of symbols and rules for their use. There are
many computer languages and a wide variety of applications for them. Program-
mers use languages to enable computers to do real work. We're providing a pilot
program written in Quick BASIC to demonstrate data communications with Watlow
controllers. You can download the MS-DOS™ version files ("comms4.zip" and
"comms4tm.zip" and com5set.exe) from the Watlow BBS, (507) 454-3958.
Syntax
Syntax for a natural language dictates how we put words together to make phrases
and sentences. In data communications, syntax also dictates how we order the
parts of a message.
ASCII
-V
+V