246 Unit Four Control Systems and Maintenance
Applications that do not involve long-distance communications
require only a few RS-232C connections. These are the signal ground
(AB), transmitted data (BA), and received data (BB), Figure 10-6. Long
distance communications (those involving modems) require many
other connections.
Another common use of the RS-232C serial port is to connect a robot to
a PC. A PC can be used offline to write and debug the initial program. The
program can then be downloaded to the robot, using the RS-232C port, to
provide the robot with the exact moves needed for an application.
When preparing cables to be used with the RS-232C port, do not exceed
a total cable capacitance of 2500 picofarads (pF). The capacitance limits
the distance between the two pieces of equipment. Another limitation is
a 20-kilobit maximum data transfer rate. Although the RS-232C standard
does not specify the data format, such as 8 bit ASCII.
Parallel Ports
Parallel transmission sends multiple bits of data at the same time,
following side-by-side paths, like the lanes of a highway. This method
allows digital equipment to communicate with peripheral equipment
using parallel ports. Although parallel transmission is faster, it is gener-
ally considered too costly to use over long distances.
One commonly used parallel port standard is the IEEE 488. It was
developed in 1975 by the Institute of Electrical and Electronic Engineers to
reduce the amount of time needed to set up test equipment. At that time,
test equipment had its own independent interface and interfacing more
than two devices to a computer could be complicated and expensive. The
IEEE 488 standard allows computer-controlled test equipment to be set up
in a matter of hours, which benefits any industry using automated testing.
The IEEE 488 standard applies when a computer is connected to
measuring equipment using a general-purpose interface bus (GPIB).
The GPIB is a cable that interfaces system controllers with program-
mable instruments, and can support a maximum of 15 devices,
Figure 10-7. Total cable length is 20 meters, or 2 meters times the
Microprocessor module
inside a robot controller
External CRT
video terminal
BA
(Transmitted data)
BB
(Received data)
AB
(Common ground)
Figure 10-6. The external video terminal is interfaced to a robot controller’s microprocessor module
via RS-232C connections. This illustration shows how AB, BA, and BB connections communicate
between the microprocessor and the video terminal.