IBM eServer zSeries 900 Technical Guide

86 IBM eServer zSeries 900 Technical Guide

Parallel channel modes

The IBM S/360, S/370 I/O interface is the communication link between a physical channel on

a System/370, System/390 or z900 server and an I/O control unit. It is designed to work in

S/370 mode, in ESA/390 mode, or in z/Architecture mode. The z900 server supports the

S/370 I/O interface in ESA/390 and z/Architecture modes only.

The parallel channel interface includes:

򐂰 Byte multiplexer channels

򐂰 Block multiplexer channels

The S/370 I/O interface uses two cables called bus and tag cables. A bus cable carries data

(one byte each way), and a tag cable carries control information. The channel physical

connection ends with bus and tag terminator plugs.

Due to the electrical characteristics of the copper cables, the maximum channel length of a

parallel channel is 122 m (400 ft). When calculating the channel length for channels with

multiple control units (daisy chaining, illustrated in Figure 3-1), a rule of thumb is to add 3 m

(10 ft) to the total cable length for every additional control unit.

There are two main parallel channel modes:

byte multiplexer and

block multiplexer. Selector

channels, which do not disconnect during an I/O operation, are no longer used and have been

replaced by block multiplexer channels.

򐂰 Byte multiplexer

Byte multiplexer channels are used with slow, unbuffered devices to transfer a small

amount of data. For each transfer, the CU must make a request to the channel in order to

be reconnected. Typically, these devices are Type-1 and Type-2 CUs.

򐂰 Block multiplexer

Block multiplexer channels are used for devices that require a large amount of data at high

speed, such as disk and tape devices (Type-3 CUs).

Two protocols are used for block multiplexer channels: Direct-Coupled Interlock (DCI) and

data streaming protocol.

The data streaming protocol, called a non-interlocked transfer method, provides a data

rate of up to 4.5 MegaBytes per second (MBps).

3.2.1 Connectivity

Following are connectivity options available in the parallel channel I/O interface environment

supported by z/Architecture and ESA/390 architecture.

z900 parallel channel features

The z900 server supports two types of parallel channel features; the 4-port parallel feature

and the 3-port parallel feature.

4-port parallel feature

The 4-port parallel feature (feature code 2304) occupies one slot in the z900 compatibility I/O

cage. The feature has four ports, with one CHPID associated with each port.

The z900 supports a maximum of 88 parallel channels on a new build, and up to 96 via

Request for Price Quotation (RPQ) 8P2198. This RPQ provides an additional compatibility

I/O cage to enable installation of additional parallel channel features. This RPQ is not required

if a G5/G6 server with more than 88 parallel channels installed is upgraded to a z900.