Communications Protocol - Advanced Energy MDX II User Manual

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Advanced Energy

COMMUNICATIONS PROTOCOL

The AE Bus communications protocol uses pure binary data (nothing is coded into ASCII, for
example), and is designed for multi-drop communications networks (networks that contain
two or more nodes). The host computer is always the master unit. Slave units transmit only
after receiving a message packet from the master, and then only to the master. The
transmission parameters are as follows:
• RS-232 or RS-485 transmission standard
• Four baud rates (selected by DIP switch):
– RS-232: 1200, 4800, 9600, 19,200
– RS-485: 9600, 19,200, 38,400, 57,600
• Odd parity
• 1 start bit, 8 data bits, one stop bit
The convention used in the MDXII unit is that a low-order byte is transmitted before higher
order bytes.
The timeout period for the MDXII unit is adjustable from 0.02 s to 5 s (see "Serial Port
Timeout Value" on page 5-17). If a transmission delay occurs, and more than the specified
time elapses between bytes, the unit will reset and begin looking for a new packet.
The host computer must finish a transaction with an MDXII unit before it initiates another
transaction (either with the same unit or with any other MDXII unit). If you have selected RS-
232, the MDXII unit sends data through Serial pin 2 (TXD); this pin must be connected to
pin 2 (RXD) on the host computer's connector. The MDXII unit receives data through Serial
pin 3 (RXD); this pin must be connected to pin 3 (TXD) on the host computer's connector.
If you are using RS-485, the MDXII unit Serial pin 6 (TR-) and Serial pin 7 (TR+) are the
transceiver lines.
What Is the Message Packet?
The AE Bus message packet combines chunks of information in such a way that much
information can be sent over communication lines at one time. Each packet contains four, or
possibly five, types of information or "fields."
• Header (contains the unit's address and the length of Data field)
• Optional (supplements the Header field)
• Command (see Table 7-9 on page 7-20)
• Data (contains parameter setting or status, Command Status Response (CSR), or nothing)
• Checksum (aids in error checking)
7-8 Serial Port Connection, Controls and Indicators 5700262-R