Protocols

To communicate with instruments and devices, there is a hardware protocol and a software protocol. These protocols describe the standard how to send and receive messages. As often hardware protocol and software protocols are used in certain combinations, both are often mixed up or used as a representative for the other one.

In this article, we will give a brief overview over commonly used protocols.

Hardware protocols

Hardware protocols are those that describe the physical transmission of a signal, e.g. electrically or optically. The description of the standard can include several things like

• the number of wires
• the connector
• the electrical signal strength and discriminator levels to identify a high or a low signal
• the possibility to do bus communication (to talk to multiple instruments at one communication line)

RS-232

RS-232 is commonly found in conjunction with COM port. It allow for a direct conversation with single instrument. Bus like communication is not supported. Instruments

RS-485

RS-485 is a transmission protocol that allows to add multiple devices to the same communication line and thus allow bus communication. It always requires an appropriate software protocol where the address of the receiver is sent.

IEEE-488

https://en.wikipedia.org/wiki/IEEE-488

USB

https://en.wikipedia.org/wiki/USB

Ethernet

https://en.wikipedia.org/wiki/Ethernet

Software protocols

In any case, a communication with an instruments relies on sending and receiving bytes, which are represented by changes of a physical quantity e.g. a voltage. The order of these bytes and their meaning is described by the software protocol. Most instrument manufacturer use sequences of bytes that can be, by using an encoding table, changed to a human readable, sense making, sequence of characters. Typically the encoding relies on the ASCII encoding table which is why an ASCII based communication protocol is synonym for a human-readable protocol. Other software protocols just use sequences of bytes that always represent a certain command but the bytes itself do not have an understandable meaning.

SCPI

The SCPI standard is a ASCII based protcol that defines the assembly of messages in a certain format. It comprises a bunch of commands that have to be implemented to comply with the standard. A typical standard command is the well known "*IDN?" that can be sent to ask for the identification string of the instrument. Other commands can be freely defined as long as they fulfill the style requirements. SCPI is most often used in confunction with GPIB/IEEE488.2 but can also be found at instruments that use RS-232.

https://en.wikipedia.org/wiki/Standard_Commands_for_Programmable_Instruments

Modbus

Modbus is a protocol to read and write memory registers. It is a protocol that supports bus communication as each message contains an address number in the range 0-255. Each value is represented by two bytes in the memory of the instruments and for each value a memory position is described that can be read and sometimes also set. 2 bytes are 16 bits that can represent numbers in the range 0 - 65535. For each value, the number of decimal positions is defined in the programming guide. For example, the number 12345 can represent the voltage 12.345 V if the value is defined to have three decimal positions. It means that both communication partners need to know the the decimal positions of each value beforehand as this information is not transmitted. Furthermore, each value can be interpreted as signed integer. In that case the two bytes correspond to a number between -32768 and 32767 including these numbers. Modbus can be most often found together with RS-485 but can also be found with Ethernet.

https://en.wikipedia.org/wiki/Modbus

CAN bus

https://en.wikipedia.org/wiki/CAN_bus

Other protcols

Some protocols are neither pure hardware nor pure software protocols that are listed here.

TCP/IP

TCP/IP is a network control that does not describe a set of available command messages and therefore is no software protocol as defined above. It is network protocol that handle the sending of message packages (frames) and to make sure these frames reach the recipient. At the same time, it is not a hardware protocol.

https://en.wikipedia.org/wiki/Internet_protocol_suite