UPD6708 datasheet(7/72 Pages) NEC

7 / 72 page

µPD6708

2.2 IEBus Communication Protocol

An overview of the IEBus is as follows.

•

Communication system: Half-duplex asynchronous communication

•

Multi-master system

All the units connected to the IEBus can transfer data to the other units.

•

Broadcast communication function (communication between one unit and multiple units)

Group broadcast communication:

Broadcast communication with group units

General broadcast communication: Broadcast communication with all units.

•

Three modes with different transfer speeds selectable.

fX = 12 MHz

fX = 12.58 MHz

Maximum Number of Transfer Bytes

(bytes/frame)

Mode 0

Approx. 3.9 Kbps

Approx. 4.1 Kbps

Mode 1

Approx. 17 Kbps

Approx. 18 Kbps

Mode 2

Approx. 26 Kbps

Approx. 27 Kbps

128

•

Access control: CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

The priority order for bus occupancy is as follows.

<1> Broadcast communication takes precedence over ordinary communication (i. e., communication between one

unit and another).

<2> The lowest master address has the highest priority.

•

Communication scale

Number of units:

MAX. 50

Cable length:

MAX. 150 m (with twisted-pair cable <Resistance: 0.1

Ω/m or less>)

Load capacity:

MAX. 8000 pF <between BUS– and BUS+>, fX = 12 MHz

MAX. 7100 pF <between BUS– and BUS+>, fX = 12.58 MHz

Terminating resistor: 120

Ω

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