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

Chapter 1 37
Introduction
Data Communications Concepts Reviewed
The second Hewlett-Packard RS-232-C implementation makes use of
additional electrical circuits defined by the standard. The cable and
connectors more closely resemble conventional RS-232-C 25-wire cable
and connectors. This implementation can be used for direct connections,
subject to the same system limitations as the three-pin version. It must
be used when modems are included (remote connection), because
additional signals are generated by modems, and required for modem
control. See the DTC Installation and Service Manual for a pin-out
diagram of the signals employed by this implementation.
The RS-422 standard is implemented by HP in a five pin version. The
connecting cable is composed of one twisted pair of wires for Data-In,
one twisted pair of wires for Data-Out, and a single wire for Signal
Ground. Devices can be connected via this interface at distances up to
1,500 meters (4,000 feet).
The coaxial cable used for the Local Area Network complies with the
802.3 standard developed by the Institute of Electrical and
Electronics Engineers (IEEE). This standard describes a
transmission access method called Carrier Sense Multiple Access with
Collision Detect (CSMA/CD).
Flow Control Protocols
Flow Control is the means by which the transfer of data between the
system and an asynchronous device is regulated. It protects both the
system and devices from data overruns. A data overrun occurs when
the sender of data transmits that data faster than the receiver can
accept it. Because the receiver cannot accept all the data being sent to
it, it is said to be overrun with data. Overruns inevitably result in lost
data.
With asynchronous devices on MPE/iX systems, flow control actually
takes place between the device and the DTC. One main method of flow
control is used: XON/XOFF protocol. Its fundamental purpose is to
protect devices from overruns.
XON/XOFF protocol is controlled by the recipient of the data being
transferred. The recipient sends an XOFF character (ASCII DC3) to the
sender of the data if, and only if, it is unable to continue to receive data.
The sender then suspends transmission and waits for the receiver to
send an XON character (ASCII DC1), thereby signaling that it is once
again ready to receive data.
The flow control method being used is sometimes referred to as the
pacing method. For instance, the terminal settings for XON/XOFF
protocol are called transmit pacing (XmitPace) and receive pacing
(RecvPace).