Figure 7-7. Example Communications Transaction - Advanced Energy MDX II User Manual

®
Advanced Energy
If the MDXII unit receives a request for data or status information, it gathers and sends the
requested information. Otherwise, it evaluates the incoming command and sends a message
packet that contains a 1-byte data value (CSR code) to the host (see Table 7-10 on page 7-57).
CSR code 0 is sent when the command has been accepted.
If the host receives a NAK from the MDXII unit, it either re-transmits the packet or does
whatever else it has been programmed to do in this situation. If the host receives an ACK, it
waits for the requested data or status information or for the CSR code that tells it whether or
not the new parameter was accepted. If the host receives no response within a reasonable
period (usually 1 s), it takes whatever action it has been programmed to take.
Meanwhile, the MDXII unit has prepared a message packet with the requested information or
appropriate CSR code, which it then transmits to the host. The host determines by means of
the checksum if the message is complete. If the host detects an error in the transmission (by
using the Checksum), it can request the packet be sent again by transmitting a NAK.
If the MDXII unit receives an ACK, it returns to the normal waiting state. If the MDXII unit
receives a NAK, it re-transmits the message packet. The MDXII unit continues to re-transmit
in response to NAK transmissions until the host stops the cycle. If the MDXII unit receives no
response, it assumes an ACK and returns to the waiting state.

Figure 7-7. Example communications transaction

7-12 Serial Port Connection, Controls and Indicators 5700262-R