Auxiliary Rs232 Interface; Gpib Interface - General Features - Ametek 7280 Instruction Manual

Chapter 6, COMPUTER OPERATION

6.3.06 Auxiliary RS232 Interface

6.3.07 GPIB Interface - General Features

6-4
The auxiliary RS232 interface allows up to sixteen model 7280s or a mixture of
compatible instruments to be connected to one serial port on the computer. The first
lock-in amplifier is connected to the computer in the usual way. Additional lock-in
amplifiers are connected in a daisy-chain fashion using null-modem cables, the AUX
RS232 port of the first to the RS232 port of the second, the AUX RS232 port of the
second to the RS232 port of the third, etc. The address of the lock-in amplifiers must
be set up from the front panel before any communication takes place. At power-up
the RS232 port of each lock-in amplifier is fully active irrespective of its address.
Since this means that all lock-in amplifiers in the daisy-chain are active on power-up,
the first command must be \N n where n is the address of one of the lock-in
amplifiers. This will deselect all but the addressed lock-in amplifier. When it is
required to communicate with another lock-in amplifier, send a new \N n command
using the relevant address.
NOTE: When programming in C remember that in order to send the character \ in
a string it is necessary to type in \\
The GPIB is a parallel digital interface with 8 bi-directional data lines, and 8 further
lines which implement additional control and communication functions.
Communication is through 24-wire cables (including 8 ground connections) with
special-purpose connectors which are constructed in such a way that they can be
stacked on top of one another to enable numerous instruments to be connected in
parallel. By means of internal hardware or software switches, each instrument is set
to a different address on the bus, usually a number in the range 0 to 31. In the model
7280 the address is set using the GPIB Settings menu or by means of the GP
command.
A most important aspect of the GPIB is that its operation is defined in minute detail
by the IEEE-488 standard, usually implemented by special-purpose semiconductor
devices that are present in each instrument and communicate with the instrument's
microprocessor. The existence of this standard greatly simplifies the problem of
programming the bus controller, i.e. the computer, to implement complex
measurement and test systems involving the interaction of numerous instruments.
There are fewer interface parameters to be set than with RS232 communications.
The operation of the GPIB requires the computer to be equipped with special-purpose
hardware, usually in the form of a plug-in card, and associated software which enable
it to act as a bus controller. The control program is written in a high-level language,
usually BASIC or C, containing additional subroutines implemented by software
supplied by the manufacturer of the interface card.
Because of the parallel nature of the GPIB and its very effective use of the control
lines, including the implementation of a three-wire handshake (see below),
comparatively high data rates, up to a few hundred thousand bytes per second, are
possible. In typical setups the data rate of the GPIB itself is not the factor that limits
the rate of operation of the control program.