AR488 GPIB Controller

The AR488 GPIB controller is an Arduino-based controller for interfacing with IEEE488 GPIB devices.

The code supports AVR based arduino boards and has been tested on Arduino Uno, Nano, Mega 2560, Micro 32U4 boards and provides a low cost alternative to other commercial interfaces.

It also supports ESP32 boards and has been tested on several ESP32 and ESP32S2 devkits. These boards come with support for wifi connection and serial over Bluetooth (not supported on ESP32S2.)

To build an interface, at least one of the aforementioned Arduino of ESP32 boards will be required to act as the interface hardware. Connecting to an instrument will require a 16 core cable and a suitable IEEE488 connector. This can be salvaged from an old GPIB cable or purchased from electronics parts suppliers. Alternatively, a PCB board can be designed to incorporate a directly mounted IEEE488 connector.

The interface firmware can optionally support the SN75160 and SN75161 GPIB transceiver integrated circuits. Details of construction and the mapping of Arduino pins to GPIB control signals and the data bus are explained in the Building an AR488 GPIB Interface section.

The interface firmware supports standard Prologix commands and adheres closely to the Prologix syntax but there are some minor differences. In particular, due to issues with the longevity of the Arduino EEPROM memory, the ++savecfg command has been implemented differently. Details of all commands can be found in the Command Reference section.

Driver Installation

Most recent OS (Windows, Linux and MacOS) will recognize the serial chipsets used by the boards listed above, so no special driver installation is required.

Firmware Upgrades

The firmware is upgradeable via the Arduino IDE or the platformio command line tool in the usual manner, however an AVR programmer can also be used to upload the firmware to the Arduino microcontroller. Pre-compiled firmwares for some boards are available from https://github.com/Twilight-Logic/AR488

Client Software

The interface can be accessed via a number of software client programs:

When using direct USB-based serial connection, terminal clients can connect via a virtual COM port and should be set to 115200 baud, no parity, 8 data bits and 1 stop bit when connecting to the interface. On Linux, the port will be a TTY device such as /dev/ttyUSB0 or /dev/ttyACM0.

For using Bluetooth or Wifi to connect to the AR488, please refer to the Remote Connection section.

Specific considerations apply when using an Arduino based interface with EZGPIB and the KE5FX toolkit. These are described in the Working with EZGPIB and KE5FX section.

Operating Modes

The interface can operate in both controller and device modes.

Controller mode

In this mode the interface can control and read data from various instruments including Digital multimeters (DMMs), oscilloscopes, signal generators and spectrum analyzers. When powered on, the controller sends out an IFC (Interface Clear) to the GPIB bus to indicate that it is now the Controller-in-Charge (CIC).

All AR488/Prologix specific commands are preceded with the ++ sequence and terminated with a carriage return (CR), linefeed (LF) or both (CRLF). Commands are sent to or affect the currently addressed instrument which can be specified with the ++addr command (see command ++help for more information).

By default, the controller is at GPIB address 0.

As with the Prologix interface, the controller has an auto mode that allows data to be read from the instrument without having to repeatedly issue ++read commands. After ++auto 1 is issued, the controller will continue to perform reading of measurements automatically after the next ++read command is used and using the parameters that were specified when issuing that command.

Device mode

The interface supports device mode allowing it to be used to send data to GPIB devices such as plotters via a serial USB connection. All device mode commands are supported.

Transmission of data

Interrupting transmission of data

While reading of data for the GPIB bus is in progress, the interface will still respond to the ++ sequence that indicates a command. For example, under certain conditions when the instrument is addressed to talk (e.g. when eos is set to 3 [no terminator character] and the expected termination character is not received from the instrument, or read with eoi and the instrument is not configured to assert eoi, or auto mode is enabled), data transmission may continue indefinitely. The interface will still respond to the ++ sequence followed by a command (e.g. ++auto 0 or ++rst). Data transmission can be stopped and the configuration can then be adjusted.

Sending Data and Special characters

Carriage return (CR, 0x0D, 13d), newline (a.k.a. linefeed) (LF, 0x0A, 10d), escape (0x1B, 27d) and + (0x2B, 43d) are special control characters.

Carriage return and newline terminate command strings and direct instrument commands, whereas a sequence of two + precedes a command token. Special care needs to be taken when sending binary data to an instrument, because in this case we do not want control characters to prompt some kind of action. Rather, they need to be treated as ordinary and added to the data that is to be transmitted.

When sending binary data, the above mentioned characters must be escaped by preceding them with a single escape (0x1B, 27d) byte. For example, consider sending the following binary data sequence:

54 45 1B 53 2B 0D 54 46

It would be necessary to escape the 3 control characters and send the following:

54 45 **1B** 1B 53 **1B** 2B **1B** 0D 54 46

Without these additional escape character bytes, the special control characters present in the sequence will be interpreted as actions and an incomplete or incorrect data sequence will be sent.

It is also necessary to prevent the interface from terminating the binary data sequence with a carriage return and newline (0D 0A) as this will confuse most instruments. The command ++eos 3 can be used to turn off termination characters. The command ++eos 0 will restore default operation. See the command help that follows for more details.

Receiving data

Binary data received from an instrument is transmitted over GPIB and then via serial over USB to the host computer PC unmodified. Since binary data from instruments is not usually terminated by CR or LF characters (as is usual with ASCII data), the EOI signal can be used to indicate the end of the data transmission. Detection of the EOI signal while reading data can be accomplished with the ++read eoi command, while an optional character can be added as a delimiter with the ++eot_enable command (see the command help that follows). The instrument must be configured to send the EOI signal. For further information on enabling the sending of EOI see your instrument manual.

Listen-only and talk-only (lon and ton) modes

In device mode, the interface supports both “listen-only” and “talk-only” modes (for more details see the ++lon and ++ton commands. These modes are not addressed modes and do not require a GPIB address to be set. Therefore if any GPIB address is already set, it is simply ignored. Moreover, when in either of these modes, devices are not controlled by the CIC. Data characters are sent using standard GPIB handshaking , but GPIB commands are ignored. The bus acts as a simple one to many transmission medium. In lon mode, the device will receive any data placed on the bus by any talker, including any other addressed device or controller. Since only ONE talker can exist on the bus at a time, there can only be one device in “talk-only” mode on the bus, however multiple “listen-only” devices can be present and all will receive the data sent by the talker.

Wireless communication

The AR488 interface can communicate using a Bluetooth module (HC05 or HC06). The firmware sketch supports auto-configuration of the Bluetooth HC05 module, the details of which can be found in the Configuration section and the AR Bluetooth Support supplement. Automatic configuration is not possible with a HC06 module so although this can be used to provide Bluetooth communication, it has to be configured manually.

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