UPD6708 datasheet(12/72 Pages) NEC

12 / 72 page

µPD6708

(4)

Control field

The control field indicates the type of data and the transfer direction of the subsequent data field.

The control field consists of 4 control bits, a parity bit, and an acknowledge bit.

The control bits are output from the MSB.

A parity bit is output after the control bits. When the parity is even and the slave can execute the function requested

by the master unit, the slave unit outputs an acknowledge signal, and then outputs the next message length field. If the

slave unit cannot execute the function requested by the master unit even if the parity is even, or if the parity is odd, the slave

unit does not output the acknowledge signal but returns to the standby (monitor) state.

After the master unit has confirmed the acknowledge signal, it starts outputting the next message length field.

If the master unit is cannot confirm the acknowledge signal, it enters the standby state and stops communication. In

the case of broadcast communication, however, the master unit starts outputting the message length field without confirming

the acknowledge signal.

For the functions of the control bits, see Table 2-3.

(5)

Message length field

The message length field is used to specify the number of communication data bytes.

The message length field comprises 8 message length bits, a parity bit and, an acknowledge bit.

The message length bits are output from the MSB. The message length bits indicate the number of communication data

bytes as shown in Table 2-2.

Table 2-2. Meaning of Message Length Bits

Message Length Bits (hex)

Number of Transmission Data Bytes

01H

1 byte

02H

2 bytes

FFH

255 bytes

00H

256 bytes

Remark

In the communication mode, if the number of bytes exceeding the maximum number of transfer bytes per frame

is set, two or more frames are communicated. In this case, the message length bits indicate the number of

remaining communication data bytes during the second communication and onward.

The operation of this field differs depending on whether the master transmits (bit 3 of control bits is 1) or receives (bit

3 of control bits is 0) data.

<1> When master transmits data

The message length bits and parity bit are output by the master unit. The slave unit outputs the acknowledge signal

and then the next data field if it detects that the parity is even. The slave unit does not output the acknowledge signal

in the case of broadcast communication.

If the parity is odd, the slave unit assumes that the message length bits have not been received correctly, and returns

to the standby (monitor) state without outputting the acknowledge signal. In this case, the master unit also returns to

the standby state, and communication ceases.

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