Dolby Laboratories CP650, Руководство по установке

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Dolby
®
Model CP650 Installation Manual External Control of CP650
E-5
The DDCMP Protocol
Digital Data Communications Message Protocol (DDCMP) was developed by Digital
Equipment Corporation as a serial link layer protocol.
All messages between the CP650 and the remote accessories take place within the
context of the DDCMP protocol. An important reference document,
ddcmp.txt , is
published by Digital Equipment Corporation
The network is a half-duplex multipoint network. The operating mode of the remote
accessories is that of a half-duplex tributary station, with the control station being the
CP650.
The CP650 supports remote accessory device addresses 1 through 9.
The CP650 has intentionally added one nonstandard modification to the DDCMP
protocol. If the CP650 sends out the hex value “FF” twenty times in a row, all remote
accessories should recycle themselves to their power-on state. This includes resetting
themselves to the ISTRT state, and attempting to re-establish the link.
Disclaimer: An attempt has been made to implement the DDCMP protocol as
defined in the document referenced above. If there is a difference between the
reference document and the CP650 implementation, the CP650 method is defined to
be correct, and will not be modified. Dolby Laboratories reserves the right to make
changes to the remote port communication scheme or interface at any time, without
notice.
DDCMP Timers
A real timer must exist in both the ISTRT and ASTRT states, of approximately one
second, for sending STRT and STACK messages respectively.
Establishing the DDCMP Link
On power-up, the remote accessories should be in the ISTRT state. As specified in the
DDCMP document, the link to the CP650 is established by each remote accessory
sending one (or more) STRT messages until the remote accessory receives the
STACK message (indicating that the link has been established).
The delay between repeated STRT messages should be determined by each
accessory’s address (slightly different for each address) to prevent repeated
transmission collisions.
Remote Accessories-to-CP650 Message
A periodic ACK from the CP650 will pass the select flag to each remote accessory (in
turn). If no change in the state of the CP650 is required by the selected remote
accessory, the remote simply returns the select flag via an ACK message.
If the remote has been given the select flag, and it intends to change the state of the
CP650, it will send the one-byte data message.