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Many of the communications services available for HP 3000 Series 900 systems
are provided through the Datacommunications and Terminal Controller (DTC).
There are six types of DTCs currently supported: DTC 72MX, DTC 48, DTC 16, DTC
16iX, DTC 16MX and the DTC 16RX. The DTC 72MX, DTC 48 and the DTC 16 can
support the DTC/X.25 Network Access cards that make communications over
X.25 networks possible. The DTC 16iX, DTC 16MX and the DTC 16RX do not support
the DTC/X.25 Network Access cards.
The DTC 72MX allows up to three
connector cards, plus a LAN card. Up to three DTC/X.25 Network Access cards
are supported. The DTC 48 can contain
up to six connector cards, up to three of which may be DTC/X.25 Network Access
cards. Slot 0 must contain an asynchronous connector card, but slots 1 through
5 can contain any combination of asynchronous cards and DTC/X.25 Network
Access cards. The DTC 16 can contain
up to two asynchronous connector cards and, optionally, a DTC/X.25 Network
Access card as its third card.
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NOTE: Throughout this manual, all DTCs supporting X.25 connections
will be referred to generically, unless their type is significant to the
topic under discussion.
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X.25 System-to-System Connections
Host-based X.25 system-to-system connections are supported through the
DTC/X.25 iX Network Link. This NS
3000/iX link makes it possible for you to connect a HP 3000 Series 900 system
to a public or private X.25 packet switched network (PSN).
The DTC/X.25 iX Network Link requires that you have at least one DTC/X.25
Network Access card installed in a DTC. You must also purchase and install the
X.25 iX System Access software on the
system.
Device Connections Via PAD
PAD support makes it possible for asynchronous serial devices, such as
terminals and serial printers, to be connected to the system through an X.25
network. Serial devices are connected to a PAD, or Packet
Assembler/Disassembler.
A PAD converts asynchronous character streams into packets that can be
transmitted over a packet switched network. PAD packets follow the CCITT X.25
protocol recommendations.
The PAD support provided on MPE/iX systems conforms to the 1984 version of
CCITT recommendations X.3 and X.29. Support is also provided for the 1980
recommendation. Packet sizes of 128, 256, and 512 bytes are supported. Access
is provided for users of asynchronous serial devices connected to both public
and private PADs.
A public PAD is a service provided by a Public Data Network
(PDN), such as TELENET or TRANSPAC. Public PADs can be used with either a
dial-up or a leased line.
A private PAD is one that is connected to a PDN as a host node having
its own X.25 network address, but behaves as a PAD when connected to another
node. Private PADs require leased lines to the network. Hewlett-Packard
provides several cluster controllers for use as private PADs.
All devices connected via PAD must support XON/XOFF protocol and must be
configured to match the speed of the PAD and modem.
Only character mode and VPLUS applications are supported on PAD-connected
devices. There are also some aspects of the way transmission occurs over PAD
connections that cause differences in the way certain device control
operations take place. See the Asynchronous Serial Communications
Programmer's Reference Manual for a summary of these differences.
Figure 1-1 "X.25 Network Connections"
shows both PAD and X.25 system-to-system connections for HP 3000 Series 900
systems employing host-based network management.
Figure 1-1 X.25 Network Connections
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![[X.25 Network Connections]](img/gfx1.gif)
