4-
4
269Plus Motor Management Relay GE Power Management
THE HANDSHAKE 4 COMMUNICATIONS
4
4.5 THE HANDSHAKE
The Master always initiates the dialogue by sending a command. It also starts a response timer to see that the
slave is replying in time. This timer is a function of transmission time of the command, reply packets, plus the
time required by the 269Plus to collect information from its registers. It is determined mutually by the master
programmer and GE Power Management depending on the number of register data to be processed in one
command. The following is a table of results recorded from the 269Plus at 2400 baud:
If the reply from the slave arrives within this time, and is error free, the master considers the dialogue success-
ful and accepts the confirmation reply and any received data. Otherwise, if the reply is not in time or if it con-
tains an error or exception reply, the master takes a corrective action at the discretion of the master
programmer.
Such a corrective action could include repeating the command for this slave for a predetermined number of
times, continuing the scan with the next slave, or posting a note on the old un-updated data and then continu-
ing the scan. The 269Plus is quite flexible in this regard: it replies when commanded, regardless of the com-
mand being an original query or a repetition.
Every time the master attempts a new dialogue to a new slave or repeats the command to the same slave, it
should observe a 3.5 character silent time. The slaves detect the silent time and reset themselves for a new
dialogue.
If a slave detects a communication error in the master command (CRC), it does not provide a reply back. This
way the master times out and repeats the command, if it is so programmed. However, if the master command
contains functional errors such as wrong function op-code, wrong register address or out of range data is
detected, it provides an error code to the master to invoke corrective action. The error messages and excep-
tion replies are discussed later in the document.
Table 4–1: HANDSHAKE DATA
BYTES READ MIN.
RESPONSE
TIME (ms)
AVG.
RESPONSE
TIME (ms)
MAX.
RESPONSE
TIME (ms)
2 6675117
10 78 82 123
250 206 222 242
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