Grass Valley 2200 - Manual (Page 21)

2-7
Status and Data Replies
Status and Data Replies
The editor interface does not respond to the controlling device upon the receipt of
each and every character. It does respond to the last character of any
complete
message. The Þrst character of any message is the Òbyte countÓ byte, specifying
the number of message bytes that are to follow. A message transmission is
complete when the total number of characters speciÞed by that byte plus that byte
itself have been received.
There are Þve possible replies in response to the complete message as outlined in
Table 2-1.
Once the interface is placed into the Select State, the most common response is one
of the two-byte status reply messages. The second byte of this message is the
response to the command. The upper two bits signify whether the command was
accepted or not as illustrated in the following Þgure:
Note that these bits are mutually exclusive; that is, both may not be set in the same
message.
If Command Accept is true, the message was forwarded to its handler for
execution. However, this status reply only means that the command was received
with no transmission errors and passed the protocol validation test (byte count
and command code) for the message structure itself. The message may still not be
executed by the Switcher for failures on the Protocol Command level.
Table 2-1. Possible Command Responses
Response Type Message In Response To:
Acknowledge 0x84 Valid Select or Poll Address
Negative
Acknowledge 0x85 Invalid Select or Poll Address; Line Error
Protocol Error 0x01 0x40 Invalid Protocol Message (Read or Write);
Timeout Error in Select State
Command Accepted 0x01 0x80 Valid Protocol “Write” Message
Data Message Write Message
(length varies) Valid Protocol “Read” Message
Bit Bit
7 6 5 4 3 2 1 0
X X 0 0 0 0 0 0
1 = Protocol Error
1 = Command Accepted