Two dierent procedures for data transmission can be used, either with or without a block check
character:
• data transmission without a block check character: 3964
• data transmission with a block check character: 3964R
In this manual, the designation 3964(R) is used when descriptions and notes refer to both data
transmission procedures.
Performance Limits with 3964(R)
Performance Limits of the 3964(R) Procedure:
• Further processing of the send/receive data by the PLC program in the communication
partner is not guaranteed. You can only ensure this by using a programmable
acknowledgment mechanism.
• The block check of the 3964R procedure (EXOR operation) cannot detect missing zeros (as a
whole character) because a zero in the EXOR operation does not aect the result of the
calculation.
Although the loss of an entire character (this character has to be a zero!) is highly unlikely, it
could possibly occur under very bad transmission conditions.
You can protect a transmission against such errors by sending the length of the data message
along with the data itself, and having the length checked at the other end.
2.4 Data Transmission with the ASCII Driver
2.4.1 Data Transmission with the ASCII Driver
Introduction
The ASCII driver controls data transmission via a point-to-point connection between the CP 440
and a communication partner. This driver contains the physical layer (layer 1).
The structure of the message frames is left open through the S7 user passing on the complete
send message frame to the CP 440. For the receive direction, the end criterion of a message must
be congured. The structure of the send message frames may dier from that of the receive
message frames.
The ASCII driver allows data of any structure (all printable ASCII characters as well as all other
characters from 00 through FFH (with 8 data bit character frames) or from 00 through 7FH (with
7 data bit character frames) to be sent and received.
Both RS422 and RS485 operation are possible.
Basic Principles of Serial Data Transmission
2.4 Data Transmission with the ASCII Driver
Point-to-point connection CP 440 Installation and Parameter Assignment
Equipment Manual, 03/2022, A5E00057742-AE 23
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