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3. One-way Transfer Procedure
This procedure sends out data until it has all been sent and is used when
the messages are so short that answerbacks need not be checked.
For longer messages, however, the receiving device must acquire each
message in time with the transfer sequence, which inserts 20 millisec-
onds intervals.
Types of Messages
Message
Request data 1
Data set 1
Command ID
RQ1 (11H)
DT1 (12H)
•Request data 1: RQ1 (11H)
This message is sent out when there is a need to acquire data from a
device at the other end of the interface. It contains data for the address
and size that specify designation and length, respectively, of data
required.
On receiving an RQ1 message, the remote device checks its memory
for the data address and size that satisfy the request.
If it finds them and is ready for communication, the device will transmit
a “Data set 1 (DT1)” message, which contains the requested data.
Otherwise, the device won't send out anything.
Byte
F0H
41H
DEV
MDL
11H
aaH
|
|
ssH
|
|
sum
F7H
Description
Exclusive Status
Manufacturer ID (Roland)
Device ID
Model ID
Command ID
Address MSB
|
|
LSB
Size MSB
|
|
LSB
Check sum
End of exclusive
* The size of the requested data does not indicate the number of bytes
that will make up a DT1 message, but represents the address fields
where the requested data resides.
* Some models are subject to limitations in data format used for a single
transaction. Requested data, for example, may have a limit in length
or must be divided into predetermined address fields before it is
exchanged across the interface.
* The same number of bytes comprises address and size data, which,
however, vary with the Model ID.
* The error-checking process uses a checksum that provides a bit pattern
where the last 7 bits are zero when values for an address, size, and that
checksum are summed.
•Data set 1: DT1 (12H)
This message corresponds to the actual data transfer process.
Because every byte in the data is assigned a unique address, a DT1
message can convey the starting address of one or more bits of data as
well as a series of data formatted in an address-dependent order.
The MIDI standards inhibit non real-time messages from interrupting
an Exclusive one. This fact is inconvenient for devices that support a
“soft-thru” function. To maintain compatibility with such devices,
Roland has limited the DT1 to 256 bytes so that an excessively long
message is sent out in separate 'segments'.
Byte
F0H
41H
DEV
MDL
12H
aaH
|
|
ddH
|
|
sum
F7H
Description
Exclusive Status
Manufacturer ID (Roland)
Device ID
Model ID
Command ID
Address MSB
|
|
LSB
Data MSB
|
|
LSB
Check sum
End of exclusive
* A DT1 message is capable of providing only the valid data among those
specified by an RQ1 message.
* Some models are subject to limitations in data format used for a single
transaction. Requested data, for example, may have a limit in length
or must be divided into predetermined address fields before it is
exchanged across the interface.
* The number of bytes comprising address data varies from one Model
ID to another.
* The error-checking process uses a checksum that provides a bit pattern
where the last 7 bits are zero when values for an address, data, and that
checksum are summed.
•Example of Message Transactions
•Device A sending data to Device B
Transfer of a DT1 message is all that takes place.
Device (A) Device (B)
[Data set 1]
[Data set 1]
[
Data set 1
]
* More than 20m sec time internal.
•Device B requesting data from Device A
Device B sends an RQ1 message to Device A.
Checking the message, Device A sends a DT1 message back to Device
B.
Device (A) Device (B)
[Data set 1] [Request data]
[Data set 1]
[
Data set 1
]
* More than 20m sec time internal.
[Data set 1]