Grass Valley Switcher Products - Manual (Page 46)

46 Switcher Products — Protocols Manual
Section 2 — Editor Protocol
Status Replies
The switcher responds to a command by sending a two-byte status
message to the editor. The first byte is the byte count which is 01H, and the
second byte is the switcher’s response to the command.
The two most common responses are 80H (command accepted) and 40H
(protocol error) as shown in (
Figure 6 ).
Figure 6. Status Reply Message
The status bits are described below. Note that the Command Accept bit and
Protocol Error bit cannot both be set.
• If Command Accept is true, the command is forwarded to the specified
effects system. Only the Command Code Byte, Byte Count Byte, and the
Effect Address Byte are checked for validity. It does not mean that the
switcher executed the command correctly.
• If Protocol Error is true, an illegal function may have been requested.
Error Detection
Error detection is a normal part of the communication process. Communi-
cation or “handshaking” errors can take many forms, such as parity errors,
framing errors, or data overrun errors. When the Editor protocol detects
communication errors, it responds with a negative acknowledge byte of
85H and then aborts all communications. Any remaining un-executed com-
mands are discarded after the error indication occurs.
In order to resume communication, the editor must transmit a break/
address sequence to return the channel to the Select state. If an error is
detected during the receipt of the tributary address, the Editor protocol
drops back into the Idle state without transmitting the 85H Negative
Acknowledge byte.
In general, it is preferable for the editor to wait until it receives the
Command Accept response before sending a new command, although it is
possible to send a continuous stream of commands without waiting for
replies after each one.
7 6 5 4 3 2 1 0
Bit Bit
1 = Protocol Error
1 = Command Accepted
X X 0 0 0 0 0 0
8063_02_03_r0