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CHAPTER 17
IEBus CONTROLLER (
PD178096A, 178098A, 178F098 ONLY)
User’s Manual U12790EJ2V0UD
(6) Telegraph length field
This field is output by the transmission side to inform the reception side of the number of bytes of the transmit
data.
The configuration of the telegraph length field is as shown in Figure 17-5.
Table 17-6 shows the relationship between the telegraph length bit and the number of transmit data.
Figure 17-5. Telegraph Length Field
MSB
LSB
Telegraph length field
Telegraph length bit (8 bits)
Parity ACK
Table 17-6. Contents of Telegraph Length Bit
Telegraph Length Bit (Hex)
Number of Transmit Data Bytes
01H
1 byte
02H
2 bytes
||
FFH
255 bytes
00H
256 bytes
The operation of the telegraph length field differs depending on whether the master transmits data (when control
bit 3 is 1) or receives data (when control bit 3 is 0).
(a) When master transmits data
The telegraph length bit and parity bit are output by the master unit. When the slave unit detects that the
parity is even, it outputs the acknowledge signal, and starts outputting the data field. During broadcast
communication, however, the slave unit does not output the acknowledge signal.
If the parity is odd, the slave unit judges that the telegraph length bit has not been correctly received, does
not output the acknowledge signal, and returns to the standby (monitor) status. At this time, the master unit
also returns to the standby status, and communication ends.
(b) When master receives data
The telegraph length bit and parity bit are output by the slave unit and the synchronization signals of bits
are output by the master unit. If the master unit detects that the parity bit is even, it outputs the acknowledge
signal.
If the parity bit is odd, the master unit judges that the telegraph length bit has not been correctly received,
does not output the acknowledge signal, and returns to the standby (monitor) status. At this time, the slave
unit also returns to the standby status, and communication ends.