The Twinwright controller

The Twinwright controller (kernel) is Twinwright's own CNC controller firmware. In Studio it runs the twin's machine. The same code is built for the MC-1's controller board.

It speaks the Grbl 1.1 serial protocol, so G-code senders work with it. Settings, alarms and errors keep the numbers senders know.

What it does

• Motion: look-ahead planning over 100 blocks, corner speeds from $11, and arcs on the true circle. Motion runs in 1 ms slices. Each axis keeps to its maximum rate ($110–$112) and acceleration ($120–$122).
• Machine functions:
  • homing, hard and soft limits, probing and the tool setter;
  • the spindle drive (VFD) over Modbus, and coolant;
  • tool changes as $341 sets them;
  • the safety door with parking;
  • overrides, jogging and the handwheel.
• Programs on the controller: programs on its file system run with O-word subroutines and macros (see O-words).
• G-code: RS274/NGC and LinuxCNC. This includes a tool table, cutter radius compensation, path blending, expressions and parameters (see G-code and M-code).

The serial protocol, the real-time commands and the $ commands are in Protocol. Every setting is in Settings, every alarm in Alarm codes, and every error in Error codes.

Compared with other Grbl controllers

Senders written for Grbl work with the controller. This section lists what you notice when you are used to another Grbl controller.

Where the controller stops

In these cases some Grbl controllers carry on; the Twinwright controller stops or raises an alarm.

• The spindle drive stops by itself (a trip, an overload, or its own stop key): the feed stops within 0.2 s, and the controller raises ALARM:14.
• The spindle drive does not answer: the axes slow down to a stop first, so the position is kept. Then the controller raises ALARM:19.
• Tool change:
  • The spindle always stops for a tool change.
  • If a tool change goes on without a confirmation, the spindle and coolant come back as they were.
• Resume after a feed hold or the door: the spindle reaches its speed before the tool moves.
• The door opens again while the machine returns from parking: the machine stops and rises to the parking height again. It waits for the next cycle start.
• The sender is lost (USB unplugged, the port closed): the job stops. $1030 controls this; it is on by default.
• Probe and homing switches: glitches on the wires do not count as a touch.
• UTF-8 text in a program (Chinese, "Ø", "–") is read as text, never as a real-time command.
• An R-form arc with no chord in its plane, for example G91 G2 X0 Z1 R5, answers error:33.

More G-code

Some Grbl controllers refuse or ignore these; the Twinwright controller runs them as the standards define them:

• the tool table (G43 Hn, G43.2, G10 L1/L10/L11);
• cutter radius compensation (G41, G42);
• path blending (G64 P) and exact stop (G61.1);
• O-word subroutines and control flow, # parameters and expressions;
• G52, polar coordinates (@, ^) and M99;
• G85, G86 and G89 drill and bore, where some Grbl controllers accept them and do not move;
• what CAM post-processors often write: a % line with a name, G49 H0, Fanuc program number lines (O1000) and Centroid's M25.

Closer to the program

• Arcs follow the true circle. Some Grbl controllers cut them into chords up to 4 × $12 off the circle.
• Arcs in inverse time (G93) take the programmed time. Some Grbl controllers run them several times faster.
• Dwells (G4) last as long as programmed. Some Grbl controllers round them to 50 ms steps.
• Peck drilling: each hole gets the pecks it needs. When the depth is a whole number of pecks, some Grbl controllers drill one more full peck from the retract height.
• Large arcs near the soft limits do not raise false alarms.
• M72 restores the tool length offset that M70 or M73 saved. Some Grbl controllers keep a later offset when the saved one was 0 in the same offset mode (G43.1), so after M72 the same program can run there with another tool length offset.
• Acceleration: every axis keeps to $120–$122, also on small arcs. A program with very many small arcs can take up to about 2 % longer than on some Grbl controllers. An ordinary program takes about 0.1 % less.

Protocol details

• Status reports:
  • In a feed hold or with the door open, the feed in FS: is 0: the axes stand.
  • The distance to go is reported until the axes stop.
  • During a critical alarm, Bf: shows the planner empty.
  • With the parser state on in $10, [GC:] comes once per change.
• Input checks:
  • A bad setting number such as $45x answers error:3.
  • An integer setting written as $1=25.0 stores 25; some Grbl controllers store 250.
  • M6 and a motion in one line answer error:40.
  • G65 or G66 with other commands in one line answer error:21.
• Alarms and start-up:
  • There is no 100 ms pause after an alarm.
  • The first start with empty settings storage does not answer error:7.
• Texts and settings:
  • The descriptions of settings, alarms and errors are Twinwright's own.
  • The controller has settings of its own: $1030 (stop when the sender is lost), and $1000 and up for input shaping (off by default).
• $39=0 (legacy real-time characters off): ?, ! and ~ are text inside $ lines and comments, and commands everywhere else. The controller decides from the bytes as they arrive. So it answers ? also with the door open or in a feed hold, where some Grbl controllers sometimes do not.

Not supported

• G65 P7 (Modbus messages).
• Fanuc system variables. #4120 and the like are ordinary parameters and read 0.
• Threading (G33, G76, G33.1) without a spindle encoder. The MC-1 has none, so these answer error:20.
• Lathe roughing cycles (other Grbl controllers have none either).
• Networking, Trinamic drivers and other makes of spindle drive, which some Grbl controllers add as plug-ins.

Senders

The senders that were checked, and their settings, are in G-code senders. To compare a real machine with its twin, see Real machine and shadow mode.