Rockwell Automation 2755-DM9, -DM9E High Speed Decoder - Manual (Page 119)

Chapter 8
Host Commands Using the RS-485 Interface
8–7
Unprotected Read Reply Format
The decoder’s reply to a read command has the following structure:
UNPROTECTED READ REPLY
STATUS CODE 01000001
TRANSACTION WORD
UP TO 124 BYTES OF DATA
HI Byte LOW Byte
The following are descriptions of the decoder’s reply to a read command:
REPLY (Byte 0) - The reply byte has the following structure:
0 1 0 0 0 0 0 1
MSB LSB
0001 =
Unprotected
Read
Bit #6 Set to 1
For Reply
Always Set to 0
STATUS (Byte 1) - Refer to Table 8.A for an explanation of the status codes.
If the returned status code is 00 (hexadecimal), an entire packet of data was
removed from the decoder’s buffer. A status code of 80 (hexadecimal)
indicates that only part of a data packet was read.
Note: If only part of the data was read (status code 80 hexadecimal
received), the entire packet of data will remain at the decoder’s host port
buffer. The remainder of the data can be read by using another read
command that has an address that is the number of bytes received greater
than the previous address:
First read command address = X
N = data bytes received with an 80 (hexadecimal) status code; indicating
data was truncated.
Send another read command with an address of X + N