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61
MIDI Implementation
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.’
• 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, or 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.
fig.MessageTransaction1.eps
• 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
fig.MessageTransaction2.eps
Byte
Description
F0H
System Exclusive Status
41H Manufacturer ID (Roland)
DEV Device ID
MDL Model ID
12H Command ID
aaH Address MSB
: :
: :
Address LSB
ddH Data MSB
: :
: :
Data LSB
sum Check sum
F7H EOX (End of System Exclusive)
Device (B)Device (A)
Data Set 1
* Wait for 20 msec or longer.
Data Set 1
Data Set 1
Device (B)Device (A)
Data Set 1 Request Data
Data Set 1
* Wait for 20 msec or longer.
Data Set 1
Data Set 1
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