Casio LK-270 - Manual (Page 31)

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19.3.16 img : Parameter Set Image
For a bulk data transfer operation, the parameter set data to be transferred is read sequentially in 16-bit units starting from
the top address. Read values are divided into 3-byte segments as shown below, and then sent in sequence.
The following is the conversion format, which is the same as the individual parameter 16-bit transfer detailed above.
16-bit Memory Image
If the length of the data being transferred is an odd number of bytes, the final bit field is filled in with zero.
Parameter Set Packet Division
The maximum image size of the parameter set that can be sent with one packet is 128 bytes. When data that exceeds 128
bytes is transferred, data is always divided into 128-byte units, except for the final packet. Because of this, the data in
received packets can be stored in an areas the size of [Parameter Set Start Address] + [Packet Number]
× 128.
For bulk parameter set transfer, the 16-bit image is divided in 3-byte groups, so the img field length is the value produced
by multiplying the len value (len + 1 when len is an odd number) by 3/2. Note, however, that a parameter set of 128 bytes
or less can be sent using a single packet, and anything greater than 128 bytes is divided among multiple packets. This
means that the maximum length "data" field is 128
× 3/2 = 192 bytes.
Only one parameter set can be transferred per session, and data from different parameter sets cannot be mixed within a
single packet, even when sending multiple parameter sets. Different parameter sets are always divided into separate packets.
19.3.17 sum : Check Sum
In this case, this "sum" field contains a value, which, when added to the total value of the "img" field, makes the lower
seven bits 0.
The receiving side checks if this is true, and performs error handling (re-request, etc.) if it is not.
19.3.18 EOX : End of System Exclusive Message
(End of System Exclusive message Status = F7H)
Format: 0dddddddB 0cccccccB 000000abB
MSB: abccccccB
LSB: cdddddddB
↓
data0: 0dddddddB
data1: 0cccccccB
data2: 000000abB
Format: 0sssssssB
Format: 11110111B