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MIDI Implimentation
Example of an Exclusive message and calculating
a checksum
Roland Exclusive messages (RQ1, DT1) are transmitted with a checksum at the end
(before F7) to make sure that the message was correctly received. The value of the
checksum is determined by the address and data (or size) of the transmitted
Exclusive message.
How to calculate the checksum (hexadecimal numbers are
indicated by "H")
The checksum is a value derived by adding the address, size, and checksum itself
and inverting the lower 7 bits.
Here's an example of how the checksum is calculated. We will assume that in the
Exclusive message we are transmitting, the address is aa bb ccH and the data or
size is dd ee ffH.
aa+bb+cc+dd+ee+ff = sum
sum ÷ 128 = quotient ... remainder
128 - remainder = checksum
<Example 1> Set the UM-880's Memory 3 USB 1 Merge setting to "Enable"
According to the Parameter address map (p. 53), the address for Memory 3 USB 1
Merge is 03 08H, and for Enable the value of the parameter should be 01H. Thus,
F0
41
10
00
49
12
03 08
(1)
(2)
(3)
(4)
(5)
(6)
address
(1) Exclusive Status,
(2) ID (Roland),
(4) Model ID MSB,
(5) Model ID LSB,
(7) End of Exclusive
Next, we calculate the checksum.
03H+08H+01H = 3 + 8 + 1 = 12 (sum)
12 (sum) ÷ 128 = 0 (quotient) ... 12 (remainder)
checksum = 128 - 12 (remainder) = 126 = 74H
This means that F0 41 10 00 49 12 03 08 02 74 F7 is the message we transmit.
<Example 2> Request data transmission for memory 7
According to the Parameter address map (p. 53), the address is 0A 00H and the
size is 00 07H. Thus,
F0
41
10
00
49
11
0A 00
(1)
(2)
(3)
(4)
(5)
(6)
address
(1) Exclusive Status,
(2) ID (Roland),
(4) Model ID MSB,
(5) Model ID LSB
(7) End of Exclusive
Next we calculate the checksum.
0AH + 00H + 00H + 07H = 10 + 0 + 0 + 7 = 17 (sum)
17 (sum) ÷ 128 = 0 (quotient) ... 17 (remainder)
checksum = 128 - 17 (remainder) = 111 = 6FH
This means that F0 41 10 00 49 11 0A 00 00 07 6F F7 is the message we transmit.
56
02
??
F7
data
checksum
(7)
(3) Device ID (17),
(6) Command ID (DT1),
00 07
??
F7
data
checksum
(7)
(3) Device ID (17),
(6) Command ID(RQ1),
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