Thermocycler G-Code Concepts

G-codes are machine-readable instructions used to control hardware. You do not need to understand or use G-codes to operate a Thermocycler with Flex or OT-2. This section is a technical resource for developers building custom applications that work with the Thermocycler.

G-Code command syntax¶

Thermocycler G-codes are strings starting with M followed by an integer. Arguments are formatted as letter-number combinations without separators. Each command must end with a new line character; Opentrons modules do not support multiple commands on a single line. For a complete list of codes and examples see Thermocycler Module G-Codes.

Syntax: MCOMMAND [ARGUMENT-KEY][ARGUMENT-VALUE] TERMINATOR
Examples: M115 \n or M104 S95 H120 \n

The following table explains these G-code command elements.

Element	Description
`MCOMMAND`	This is a G-code command itself. For example, sending a simple looking command like `M115 \n` to the Thermocycler returns its serial number, model, and firmware version.
`ARGUMENT-KEY`	An optional key-value pair used to pass parameters to a command. Typically, the key is a single, uppercase letter followed by its value (usually an integer). For example, sending `M104 S95 H120 \n` sets the Thermocycler temperature to 95 °C and holds it for 120 seconds. In this command: `S` and `H` are the keys for "set temperature" and "hold time." `95` and `120` are the values for °C and seconds.
`TERMINATOR`	Every G-code sequence ends with a terminator, which is always a newline character (`\n`).

G-Code response syntax¶

Every Opentrons module returns a response after executing a command. A response will be one of the types defined below.

Response type	Description
Acknowledgement	An acknowledgement returns `OK` when the command is successful. This response type does not echo the command code.
Response	Some responses echo the command code and append `OK` to the response string when the command is successful. For example, successfully sending the command `M119` (get the Thermocycler lid and seal motor status) would return `M119 Lid: open Seal: retracted OK`. Other responses do not echo the command code and only return an acknowledgement (`OK`) or an error code.
Error	An error indicates a command failed and does not echo the command code. Error responses are strings formatted as `ERRNNN:error`, where `N` is an integer followed by a plain text description. For example, sending too many commands to a module too quickly might result in the response `ERR004:G-code cache full`. See the Thermocycler Error Codes section for a complete list.

Connection parameters¶

To establish a serial USB connection, your application must specify the baud rate and identify the module using its Vendor ID (VID) and Product ID (PID).

Baud rate: Defines the communication speed for the module.
VID: Identifies the manufacturer of the integrated circuit running the module's firmware.
PID: Identifies the specific Thermocycler type based on its model generation.

The following table lists these required connection parameters.

Module	Baud Rate	VID	PID
Thermocycler GEN1	`115200`	`0x04D8` or `0x239a`	`0xED8C` or `0x800b`
Thermocycler GEN2	`115200`	`0x0483`	`0xED8D`

Making Good G-Code Connections

Identifying modules by their VID and PID helps you write resilient, cross-platform code. Hard coding for a specific port (e.g., COM3 for Windows or /dev/ttyUSB0 for macOS) makes code brittle. By using a library like pyserial to scan for specific VID/PID combinations, your application can dynamically find and connect to the correct Opentrons module regardless of its physical connection point.