KEP MMI-1500 Installation & Operation Manual page 262

16.25.4 Method of Operation
Communications is controlled by software flow control methods. All communications begin with an STX command being
sent. The receiving device then responds with ACK if ready to accept information or NAK if busy. When communications are
ended, an ACK or NAK confirms if the data was received properly.
For Example:
Good Communication
Sequence
Host: ---> STX
MMI(Slave) ---> ACK
Host: ---> String of data
HMI ---> ACK
16.25.4.1 Word Write Operation
---> STX
<--- ACK
01 (Hexadecimal)
Word Address (Low byte)
Word Address (High byte)
Number of bytes
Data bytes
:
:
:
DLE
ETX
Basic Checksum (BCC)
<--- ACK
Note: There is no Read command. It is the responsibility
of the controlling computer to update all of the HMI units
by writing data to them. The HMI units can write data back
to the controlling computer. The controlling computer
cannot read data out of the HMI units nor can the HMI
units read data out of the controlling computer.
Delayed Communication
Sequence
Host: ---> STX
MMI(Master) ---> NAK
Host: ---> waits a while then
sends STX
MMI(Master) ---> ACK
Host: ---> String of data
HMI ---> ACK
Bad Communication
sequence
Host: ---> STX
MMI(Slave) ---> ACK
Host: ---> String of data
HMI ---> NAK
Host: repeats sequence
hoping for better results
16.24.4.2 Bit Write Operation
---> STX
<--- ACK
02 (Hexadecimal)
Bit Address (Low byte)
Bit Address (High byte)
Number of bytes
Data 00 or 01
DLE
ETX
Basic Checksum (BCC)
<--- ACK
Note: If DLE (10 hex) is in the data being sent, it must
be sent twice to avoid premature end of transmission.
Note: BCC is the XOR of all transmitted data in a frame
including the DLE and ETX commands.
Hexadecimal Codes
Code Hex
STX 02
ETX 03
ACK 06
DLE 10
NAK 15
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