IBM eServer zSeries 900 Technical Guide

Chapter 3. Connectivity 85

– If you define multiple paths from the same CHA, distribute paths across different

channel cards.

z900 servers automatically balance installed channel cards across all available SAPs. The

processor attempts to assign an equal number of each channel card type (ESCON, FICON,

etc.) in each available SAP. While all channels on a given I/O card will always be in the same

SAP, it is not predictable which I/O cards will be assigned to which SAPs. IBM recommends

you configure for normal RAS as described previously, and the machine will balance the

channels across your available SAPs.

3.2 Parallel channel

This section describes the parallel channel features, modes, and topologies available with the

z900 server.

Included is basic information on the IBM System/360 and System/370 I/O interface, also

called S/360, S/370 Original Equipment Manufacturer's Information (OEMI) interface, parallel

I/O interface, or bus and tag channel interface.

IBM introduced the parallel channel with System/360 during the early 1960s. The I/O devices

were connected using two copper cables called

bus and tag cables. Figure 3-1 shows bus

and tag cables connected sequentially to the control units. This is often referred to as “daisy

chaining.” The last CU is equipped with channel terminator blocks.

Figure 3-1 Parallel channel connectivity

The architectural limit to the number of control units in the chain is 256, but electrical

characteristics restrict the chain to a maximum of eight control units on a channel.

Daisy chaining better utilizes a channel that has slow control units attached, but a single

failing control unit or a bad cable connection can influence other control units chained on the

channel.

Terminators

Bus

Ta g

Bus

Tag

Control

Unit

Control

Unit

Control

Unit

Control

Unit

Device

Device

Device

Device

Parallel

Channel

z900 server