Rs232 And Gpib Operation; Introduction; Rs232 Interface - General Features - Ametek 7280 Instruction Manual

Chapter 6, COMPUTER OPERATION

6.3 RS232 and GPIB Operation

6.3.01 Introduction

6.3.02 RS232 Interface - General Features

6-2
Control of the lock-in amplifier from a computer is accomplished by means of
communications over the RS232 or GPIB interfaces. The communication activity
consists of the computer sending commands to the lock-in amplifier, and the lock-in
amplifier responding, either by sending back some data or by changing the setting of
one of its controls. The commands and responses are encoded in standard 7-bit
ASCII format, with one or more additional bits as required by the interface (see
below).
The two ports cannot be used simultaneously, but when a command has been
completed, the lock-in amplifier will accept a command at either port. Also when the
test echo facility has been activated all output from the computer to the GPIB can be
monitored by a terminal attached to the RS232 connector.
Although the interface is primarily intended to enable the lock-in amplifier to be
operated by a computer program specially written for an application, it can also be
used in the direct, or terminal, mode. In this mode the user enters commands on a
keyboard and reads the results on a video screen.
The simplest way to establish the terminal mode is to connect a standard terminal, or
a terminal emulator, to the RS232 port. A terminal emulator is a computer which runs
special-purpose software that makes it act as a terminal. In the default (power-up)
state of the port, the lock-in amplifier sends a convenient prompt character when it is
ready to receive a command, and echoes each character that is received.
Microsoft Windows 95/98 includes a program called HyperTerminal, usually to be
found in the Accessories group, which can be used as a terminal emulator.
Alternatively a simple terminal program with minimal facilities can be written in a
few lines of BASIC code (see appendix C.1).
The RS232 interface in the model 7280 is implemented with three wires; one carries
digital transmissions from the computer to the lock-in amplifier, the second carries
digital transmissions from the lock-in amplifier to the computer and the third is the
Logic Ground to which both signals are referred. The logic levels are ±12 V referred
to Logic Ground, and the connection may be a standard RS232 cable in conjunction
with a null modem or, alternatively, may be made up from low-cost general purpose
cable. The pinout of the RS232 connectors are shown in appendix B and cable
diagrams suitable for coupling the instrument to a computer are shown in appendix
D.
The main advantages of the RS232 interface are:
1) It communicates via a serial port which is present as standard equipment on
nearly all computers, using leads and connectors which are available from
suppliers of computer accessories or can be constructed at minimal cost in the
user's workshop.