UPD6708 datasheet(10/72 Pages) NEC

10 / 72 page

µPD6708

2.3 Transfer Protocol

The IEBus transfer signal format is shown in Figure 2-1.

Data is transferred as a series of signals called a communication frame. The number of data that can be transferred in

one communication frame and the transfer speed differ depending on the communication mode.

Figure 2-1. Transfer Signal Format

P: Parity bit (1 bit)

A: Acknowledge bit (1 bit)

When A = 0: ACK

When A = 1: NAK

N: Number of data bytes

Remark

In broadcast communication, the value of the acknowledge bit is ignored.

(1)

Header

A header comprises a start bit and a broadcast bit, as described below.

<1> Start bit

The start bit is a signal which tells the other units that data transmission will start.

The unit which is about to start transmitting data will output the low signal (the start bit) for a specified time, and then

outputs the broadcast bit.

If another unit is already outputting a start bit before one unit outputs a start bit, the unit will not output the start bit.

It will wait until the another unit completely outputs the start bit, and then outputs the broadcast bit.

The units other than the one that has started transmission detect this start bit and enters the reception state.

<2> Broadcast bit

The broadcast bit distinguishes between broadcast communication and ordinary communication.

When this bit is ‘0’, it indicates broadcast communication; when it is ‘1’, it indicates ordinary communication. There

are two types of broadcast communication: group broadcast and general broadcast. These types are identified by the

value of the slave address (for the slave address, see (3) “Slave address field”).

In broadcast communication, there are a number of slave units. Therefore, the acknowledge bit is not returned in the

fields described in (2) below and onward.

If two or more units start to transmit a communication frame simultaneously, broadcast communication takes

precedence over ordinary communication, and wins in the arbitration.

Field Name

Number of Bits

Transfer Time

Mode 0

Mode 1

Mode 2

Header

Master

Address Field

Slave Address

Field

Control Field

Message

Length Field

Data Field

112

Start

Bit

Broad-

cast

Bit

Master

Address

Slave

Address

A Control

Bit

Message

Length

Bit

Data

Bit

Data

Bit

Approx. 7330 s

Approx. 2090 s

Approx. 1590 s

Approx. 1590

× N s

Approx. 410

× N s

Approx. 300

× N s

(fx = at 12 MHz)

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