Checksum Protocol (For Data Security); Figure 3-4 Request Format For Checksum Protocol - Honeywell UDC2300 Product Manual

3.7

Checksum Protocol (for Data Security)

Introduction
Failure to use checksum protocol could make the undetected error rate for the RS422/485 link unacceptable
for your process control application.
Using checksum
protocol
Figure 3-4 Request Format for Checksum Protocol
Address
Protocol Field
UDC Status
Data Type Field
Operation Dependent Data
CHECKSUM
Carriage Return
Line Feed
10/00 RS422/485 ASCII Communications Option Product Manual
The optional Checksum Protocol is used to increase security on the
RS422/485 link. This protocol enables both your computer and your UDC
to detect messages that the RS422/485 link has transmitted inaccurately.
Thus, this protocol makes the RS422/485 communications link more
reliable.
You can use the checksum protocol with any message exchange. The
UDC uses the protocol to check the transmission of request messages.
Your computer uses the protocol to check the transmission of response
messages.
When a message exchange includes checksum protocol:
• Your UDC can tell, with high probability, if the ASCII code in the
request message has changed during transmission from your computer.
• Your computer can tell, with high probability, if the ASCII code in the
response message has changed during transmission from the UDC.
To use Checksum Protocol, you change the format of the request message
as shown in Figure 3-4 as follows:
• You use a 4204 in the request format.
• You insert a 2-digit Hexadecimal number that represents the checksum
that you have calculated from the ASCII codes in the request message
as explained in "Calculating the Checksum". See Section 7 for an
ASCII Conversion table and a Hexadecimal Binary table.
ASCII code for these characters is added
to calculate the checksum
4 2 0 4
,
,
,
,
CR LF
,
23085
27