Industrial and Scientific Computing, Validation and Automation
LabVIEW, TestStand, LabWindows/CVI

Test bench and Measurement freelance Engineer

SCPI (Standard Commands for Programmable Instruments) is a standard that defines a language for communicating with instruments based on messages. The majority of the GPIB instruments use the SCPI but also RS-232, Ethernet, VXI, PXI may use it. Several measurement instrument manufacturers have adopted this standard as HP / Agilent for example, allowing some communication uniformity between several devices.

Some examples of SCPI commands:

Instrument identification querry: *IDN? Instrument reset: RST Measure querry: MES?

The AK protocol define a communication protocol over serial RS-232 or TCP/IP connection between devices, typically between a PC and measuring instruments.

The RS-422 is a standard for serial bus. It is newer than the RS-232 and more efficient. The RS-422 cable contains a twisted pair. We can form a point to point network linking up to 10 devices (1 driver and 10 receivers) and up to 1200 meters in length. It operates in differential mode on levels +6V -6V and can reach speeds up to 10Mb/s. It can operate in half-duplex or full duplex with 4 cables.

The RS-232 is a standard for serial bus. It operates in point to point mode with one transmitter and one receiver, up to 15 meters in length. It operates with voltages of -15V to -3V for logic 1 and +3V to +15V for logic 0 (typically -12V to +12 V) and can reach speeds of 20Kb/s.

The RS-485 is a serial bus standard. It is an evolution of the RS-422 and like it, the RS-485 cable is composed of a twisted pair. We can form a point to point network linking up to 64 devices (32 transmitters and 32 receivers, this is the diference with the RS-422), and up to 1200 meters in length. It operates in differential mode on levels -6V and +6V and can reach speeds up to 10 Mbits/s. It can operate in half-duplex or full duplex with 4 wires.