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channels, which do not disconnect during an I/O operation, are no longer used and have been
replaced by block multiplexer channels.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.

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