Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)

36 Chapter1
Introduction
Data Communications Concepts Reviewed
Data Communications Concepts Reviewed
The hardware and software described in this chapter conforms to
specific rules designed to make data communications between devices
possible. To use this manual as a programming guide, you need a good
understanding of these rules and of data communications concepts in
general. If you are unfamiliar with data communications concepts, it
would be helpful for you to take a general course in data
communications before you use this manual extensively. You may also
want to review other Hewlett-Packard documentation, as listed below:
• DTC Installation and Service Manual, the hardware reference for
the DTC.
• The manual for the peripheral device.
• DTC Planning Guide.
The material in the rest of this chapter is designed to serve as a review
of several concepts that are central to successfully connecting terminals
and printers to a HP 3000 Series 900 computer.
Protocols
In order for electronic devices to communicate with each other they
must conform to well defined sets of rules which govern factors ranging
from hardware configurations to the actual process of data transfer.
Such a set of rules is called a protocol. Data communications protocols
are conventions set up to assure the orderly exchange of information
between two or more data processing entities.
Hardware Protocols
The lowest level protocols deal with the hardware connection between
two pieces of data processing equipment. The cables used to connect
asynchronous devices to the Datacommunications and Terminal
Controller conform to protocols defined by the Electronic Industries
Association (EIA). These protocols, RS-232-C and RS-422, were
developed and updated in conjunction with the Bell System and
independent equipment manufacturers.
The RS-232-C standard defines the electrical circuit functions for 25
connector pins. This standard is implemented by Hewlett-Packard in
two versions. The first version makes use of only three of the defined
transmission circuits, Data-In, Data-Out and Signal Ground. This
special implementation provides an inexpensive interface, since the
cable consists of only three wires with the connector on one end
replaced by a three-pin connector. However, it is limited to local (direct)
connections, in distances up to 15 meters (50 feet).