Hardware Reference
In-Depth Information
Photo 12.2
Prototype parallel port interface module
is greatly in excess of the rated current. Furthermore, when all devices operate
simultaneously (or within a few milliseconds of one another) the surge current
will invariably trip out the mains supply. This, in turn, causes disruption to
the manufacturing process because each device has to be individually turned
off before the mains trip can be manually reset. The client requires a simple
and reliable means of automatically sequencing the application of power to the
loads.
Specification
The time delay in applying the AC mains supply to each device is to be con-
figurable to within 1 s up to a maximum of 30 s. The operator is to be provided
with a simple graphical interface that shows the status of each load and allows
the delay to be set using a simple slider control.
Hardware
Since there are eight loads and they are only required to be switched on and off,
this application requires a simple 8-bit parallel port interface module. However,
it is expected that the production system may be expanded at some point in the
future and it could be advantageous to provide a solution that can be easily
expanded on a modular basis (see Photo 12.2).
The circuit of one 8-bit parallel interface module is shown in Figure 12.13.
The module is connected to the PC by means of a standard parallel port (see
Chapter 2). In order to cater for future expansion, the module can be assigned
to one of four controlled groups by means of a group channel select switch.
Each channel group (A , B, C, and D) will then have eight controlled channels
(channels 1-8) and each of these channels will correspond to a particular device.
