Marquee 8521 Ultra Green (HUD) User's Manual
B-4
Marquee Projectors RS-232 Control
B.5.1 $01 AND $0E (MESSAGE START AND END)
Use the
$01
control code to preface each and every transmission. This byte indicates to a network
receiver that the next byte is the first byte of a new message. Any message which (for some reason)
is partially received prior to the
$01
will be discarded.
The
$0E
control code signifies the end of each and every transmitted message. It signals to the
receiving network software that the message has been completely transmitted.
B.5.2 $13 AND $11 (STOP AND RESUME)
Normally messages can be sent to the projector before processing of earlier messages has been
completed — the projector will just store messages in a buffer until ready to process. However, if a
series of messages is sent it is possible that the projector will not be able to process them as fast as
they are being transmitted and the buffer will become full. If this happens, the projector will
immediately send the
$13
(XOFF) code to halt further transmission. This instructs the controller (and
any devices preparing to transmit) to cease transmission within three characters transmission time.
At 9600 baud (the default baud rate), this gives the controller about three milliseconds to respond.
The projector is able to accommodate the receipt of at least three more bytes after it sends
$13
(XOFF) — additional bytes may be lost. When the buffer is once again available, the projector will
send a
$11
(XON) command to resume transmission.
Note:
XON and XOFF controls apply to both directions of communication.
B.5.3 $1B
(ESCAPE)
Occasionally a byte within a message may have the same value as one of the special control codes
reserved for the transport layer. If this occurs,
$1B
must be used to "escape" the byte so that it is not
mistaken for a transport layer control code. “Escape” a byte by inserting an ESC character (
$1B
)
into the data stream just ahead of the message byte in question and then adding 128 (
$80
) to the
value of the message byte. This effectively sets the MSB (most significant bit) high, moving the value
of the byte out of the range of values reserved for transport control codes.
On the receiving end, the transport layer software recognizes the escape sequence and strips the
$1B
(ESC) from the stream. The next data byte will have its MSB reset which restores it back to its
original value.
B.6 Message
Format
The other portion of an RS-232 communication is the message itself (see Figure B-2 again). A
message portion is a sequence of up to 64 bytes — always a five-byte header followed by a variable
length body. The header supplies the information needed for routing the messages within the
projector network, and the body contains the specific projector control data. These two message
components are further described below.
B.6.1 HEADER
(NETWORK/PROJECTOR ADDRESSING)
The message header consists of three main parts:
1.
Message Length:
This byte contains a value which declares the length of the message as a
specific number of bytes. Note that message length does
not
include bytes from the transport
layer.
2.
Destination
and
Return Address (Source) Fields:
These fields contain two sixteen-bit
addresses — one for the message destination and one for the source. A breakdown of each
address field is shown in Figure B-3. Note that Near and Far Port Address bits are now
ignored — while it is strongly recommended that these bits be set to “0” to ensure
compatibility with future releases of software, it is not yet mandatory to do so. Bit #14,
however,
must
be set to 0.
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