hm2_modbus - A hostmot2 driver that implements the Modbus protocol using the PktUART ports.
loadrt hm2_modbus ports=... mbccbs=...
ports [default: <empty>]
A comma separated list of PktUART HAL names to use as Modbus hardware channel. Each must be matched with an MBCCB file specified in the mbccbs parameter. Example: ports="hm2_7i96s.0.pktuart.0","hm2_5i25.0.pktuart.2"
mbccbs [default: <empty>]
A comma separated list of "Modbus Command Control Binary" (MBCCB) file paths to use for each PktUART port as specified in the ports parameter. The path should be an absolute path to prevent nasty surprises. Example: mbccbs="/path/to/rly-and-spindle.mbccb","/path/to/lightsparks.mbccb"
debug [default: -1]
Set the message level of the running process. The message level is set if debug is set to a positive value between 0 and 5, where 0 means no messages at all and 5 means everything. A value of -1 does not touch the current message level.
Caveat Emptor: the driver must be compiled with debug messages enabled for it to start emitting messages at the debug level. Changing the message level is process-wide and all modules within the process will spit out messages at the requested level. This may cause quite some clutter in your terminal.
The hm2_modbus driver implements the Modbus protocol and maps HAL pins to Modbus devices' coils, inputs and registers. The mappings may be a complex combination of types and pins. The configuration format is described in mesambccc(1).
The Mesa FPGA board must be flashed with a PktUART capable bit-file. It is recommended to use an FPGA bit-file that supports PktUART version 3 or later. The hm2_modbus driver will run with PktUART version 2, but it will lack several important bug fixes and features, like 2 stop-bits, correct and extended inter-frame delay for high speed communication and inter-character delay measurements. Warnings will be emitted if communication settings cannot be honored by the older PktUART version. PktUART versions older than 2 are not supported and the driver will abort with an error if encountered.
The driver exports a set of HAL pins and parameters that can be used to inspect status and alter some settings in a live environment.
Up to eight instances are supported, which are instantiated by setting the ports and mbccbs parameters. The instances are named hm2_modbus.0, hm2_modbus.1, etc.. The pin names generated by the driver will always be prefixed with the driver’s name.
loadrt hm2_modbus ports="7i96s.0.pktuart.0" mbccbs="/path/to/myfile.mbccb"
The driver exports one function named process, which must be added to the servo-thread after hostmot2’s read function and before hostmot2’s write function:
addf
hm2_7i96s.0.read servo-thread
...
# Add any functions here that process data that will be sent
to
# the Modbus device(s).
...
addf hm2_modbus.0.process servo-thread
...
# Add any functions here that process data received from the
# Modbus device(s).
...
addf hm2_7i96s.0.write servo-thread
There are no software limits to how many Modbus devices may share one and the same physical bus and by extension the same hm2_modbus instance. The Modbus protocol limits the number of devices to 247 (available device IDs). However, the practical limit will be effective communication speed and bus load.
Exported parameters for driver instance N:
hm2_modbus.N.baudrate (u32, readonly)
The communication baudrate.
hm2_modbus.N.drivedelay (u32, readonly)
The transmitter wait time between enabling the transmitter and start sending in bit-times.
hm2_modbus.N.icdelay (u32, readonly)
The maximum inter-character delay accepted in received frames in bit-times. Set to zero (0) when disabled.
hm2_modbus.N.parity' (u32, readonly)
The communication parity (0=None, 1=Odd, 2=Even).
hm2_modbus.N.rxdelay (u32, readonly)
The inter-frame delay required before a packet is accepted in bit-times.
hm2_modbus.N.stopbits (u32, readonly)
The communication number of stopbits (1 or 2).
hm2_modbus.N.txdelay (u32, readonly)
The inter-frame delay inserted after a packet in bit-times.
The parameters are all read/only. There is normally no need to alter any parameters. Any tuning of values should be done in the mbccs/mbccb file.
Each driver instance N exports the following pins to control the instance’s operation and indicate the instance’s status:
hm2_modbus.N.fault (bit, output)
Indicates a fault condition.
hm2_modbus.N.fault-command (u32, output)
The command index that caused the last fault condition.
hm2_modbus.N.last-error-code (u32, output)
The errno value of the error that caused the fault condition.
hm2_modbus.N.reset (bit, input)
Reset all commands error counters and re-enable disabled commands on the rising edge of the input pin.
hm2_modbus.N.suspend (bit, input)
Suspend all activity while set. The default can be set in the MBCCB file.
Suspended start (when set in the MBCCB file) can ensure that all HAL files and commands are parsed and executed (like setp and sets commands) before Modbus communication starts. This enables you to setup scale and offset pins without bad values being pushed to the Modbus device(s). The last HAL command after setting all relevant pins and signals would be to enable Modbus communication by isssuing (replace N with actual instance number):
setp hm2_modbus.N.suspend false It may be necessary to issue a delay command as the first or last init command to ensure flushing all pins before reading pin data that will be transferred to the Modbus device(s). See also writeflush attribute in the MBCCB file.
It is possible to suspend the Modbus communication by setting the suspend pin to true in a live and running system. The suspend pin is probed every time at the end of the <commands> list and execution will become suspended if the pin is true.
Command pins
Each command in the MBCCB file (not init commands) will generate a set of pins to reflect the current state, where MM is the command number counting from zero (00):
hm2_modbus.N.command.MM.disabled (bit, output)
Set if the command is no longer sent in the commands loop.
hm2_modbus.N.command.MM.error-code (u32, output)
The errno code of the last error. The following error codes can be set:
• 5, 0x05 (EIO): The receiver detected an overrun, a false start-bit or wrong parity.
• 9, 0x09 (EBADF): The reply returned an unsupported function.
• 22, 0x16 (EINVAL): An invalid value was detected (internal error).
• 27, 0x1b (EFBIG): The received data packet size exceeds the internally allocated buffer.
• 34, 0x22 (ERANGE): The received data packet was too small or the message’s length indicator was wrong.
• 42, 0x2a (ENOMSG): The inter-character delay was too long and the packet was dropped.
• 44, 0x2c (ECHRNG): The reply had a different function than the sent function.
• 52, 0x34 (EBADE): The CRC of the received packet was wrong.
• 74, 0x4a (EBADMSG): The reply had the error-bit set.
• 75, 0x4b (EOVERFLOW): The received packet was larger than the maximum 256 bytes.
• 90, 0x5a (EMSGSIZE): The message did not fit into the maximum PDU size of 253.
• 110, 0x6e (ETIMEDOUT): The command received no reply and timed out.
hm2_modbus.N.command.MM.errors (u32, output)
The number of consecutive errors seen in this command. The command will be disabled when this count reaches five (5). The value will be reset to zero (0) when the command succeeds.
hm2_modbus.N.command.MM.reset (bit, input)
Reset this command’s error counter and re-enable the command on the rising edge of the input pin.
Each mbccb file will generate a set of pins as defined in the mbccb file. See mesambccc(1) for details.
hostmot2(9), mesambccc(1).
This man page written by B.Stultiens, as part of the LinuxCNC project.
Report bugs at https://github.com/LinuxCNC/linuxcnc/issues
Copyright © 2025 B.Stultiens
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.