Roland SPD-20 Owner's Manual page 97

fig.
* 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 Description
F0H Exclusive Status
41H Manufacturer ID (Roland)
DEV Device ID
MDL Model ID
12H Command ID
aaH Address MSB
| |
| |
LSB
ddH Data MSB
| |
| |
LSB
sum Check sum
F7H 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, size, 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
[Data set 1]
* More than 20m sec time interval.
[Data set 1]
[Data set 1]
•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
[Data set 1]
* More than 20m sec time interval.
[Data set 1]
[Data set 1]
Device B
Device B
[Request data]
1
2
3
4
5
97