Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Instrukcja Użytkownika Pobierz pdf (Strona 68)

Asynchronous Link Protocols

Chapter 4

4-25

At the end of each polling packet there is a BCC byte. At the end of each

message packet, there is either a one-byte BCC field or a two-byte CRC

field. You can select BCC or CRC through switch settings (see Chapter 3,

“Asynchronous Link Features”).

Block Check

The block check character (BCC) is a means of checking the accuracy of

each packet transmission. It is the 2’s complement of the 8-bit sum

(modulo-256 arithmetic sum) of the slave station number (STN) and all

the data bytes in the packet. For polling packets, the BCC is simply the

2’s complement of STN. The BCC does not include any other message

packet codes or response codes.

Example 1: If the master station wanted to send the data codes 08, 09,

06, 00, 02, 04, and 03 to Slave Station 20 hex (40 octal), the master

message codes would be (in hex):

DLE SOH

08 09 06 00 02 04 03 10 03 C0

DLE ETX BCCData

DLE

STX

STN

The sum of the STN and data bytes in this message packet is 40 hex. The

BCC is the 2’s complement of this sum, or C0 hex. This is shown in the

following binary calculation:

0100 0000 40 hex

1011 1111 1’s complement

1100 0000 2’s complement (C0 hex)

Example 2: To transmit the STN or data value 10 hex, you must use the

data code DLE DLE. However, only one of these DLE text characters is

included in the BCC sum. For example, to transmit the values 08, 09, 06,

00, 10, 04, and 03 hex, a slave station would use the following message

codes:

DLE STX

02 08 09 06 00 10 10 04 03 10 03 D2

DLE ETX BCCData

Represents Single Text Value of 10

1 2 ... 63 64 65 66 67 68 69 70 71 72 73 ... 234 235

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Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Instrukcja Użytkownika Strona 68