|
Half duplex
Communication scheme where data flows in both directions but in only one direction at a time.
Hard contacts
Physical switch connections.
Hard copy
Printed copy of computer information.
HDLC (High-level Data Link Control)
Standard protocol of communication oriented in message transmission (frames). The user's data field in an HDLC-frame can be of a free number of bits. The SLDC is a subset of the HDLC that defines the whole protocol in more detail and is byte-oriented.
Hexadecimal
Numbering system that utilizes base 16.
HMI (Human-Machine Interface)
The interface between the operator and the process machinery.
Host computer
One to which devices communicate. The host may download or upload programs, or the host might be used to program the device. An example would be a PLC connected to a microcomputer. The host (microcomputer) "controls" the PLC by sending programs, variables, and commands. The PLC controls the actual process but at the direction and to the specifications of the host.
Hot Swap
Exchange of components during operation.
Hysteresis
A dead band that is purposely introduced to eliminate false reads in the case of a sensor. In an encoder hysteresis would be introduced in the electronics to prevent ambiguities if the system happens to dither on a transition.
|