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FB_RS485_BUSCONTROLLER.md

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FB_RS485_BUSCONTROLLER

General

Shares one RS485 bus between the devices registered on it. It decides whose turn it is, runs that device's Modbus exchanges, and keeps the required silence between frames — both between transactions and between the steps inside one.

The unit of arbitration is a transaction: an ordered list of up to eight steps that a device hands over in one go, executed back to back with the bus held throughout. Nothing else may interleave, which is what allows a write and the read that confirms it to be one indivisible operation. See the I_RS485_DEVICE interface for the device side of the contract.

The Modbus protocol itself lives behind I_RS485_TRANSPORT, so this block never sees a CRC, a function code or a serial handle. FB_RS485_TRANSPORT_RTU is the implementation this project ships.

Turn-taking

Selection resumes at a cursor and wraps, rather than restarting at device 0. Registration order therefore does not decide service order: the device registered last is served as often as the device registered first.

Each pass sweeps every device twice — once for COMMAND work, once for POLL — so something a person or Home Assistant asked for goes ahead of routine polling, while commands are themselves round-robin so a chatty device cannot monopolise the bus.

Watch Cursor and Transactions on a running PLC to see it working.

stateDiagram-v2
    direction LR
    [*] --> Startup
    Startup --> Idle : StartupDelay elapsed
    Idle --> Idle : nobody wants the bus
    Idle --> Execute : device selected,<br/>BuildTransaction returns n > 0
    Idle --> Release : BuildTransaction returns 0
    Execute --> Collect : transport accepted the step
    Execute --> Release : watchdog
    Collect --> Gap : more steps, no abort
    Gap --> Execute : SilenceTime
    Collect --> Release : last step, or AbortOnError
    Release --> Settle : OnTransactionDone,<br/>cursor advanced
    Settle --> Idle : SilenceTime
Loading

Gap is the inter-frame silence inside a transaction — the bus is not released there. That is the difference that makes read-after-write meaningful.

Every one of those states costs at least one task cycle, which is why the RS485 task's cycle time and not the baud rate is what sets throughput. See How fast the bus goes.

Block diagram

   ┌────────────────────────┐
   │ FB_RS485_BUSCONTROLLER │
   ├────────────────────────┤
   │             BusOcupied ├── BOOL
   │           ActiveDevice ├── INT
   │                 Cursor ├── INT
   │           Transactions ├── UDINT
   │          StepsExecuted ├── UDINT
   │           StepFailures ├── UDINT
   │              Watchdogs ├── UDINT
   └────────────────────────┘

Interface

Outputs

Pin Type Description
BusOcupied BOOL TRUE until the startup delay has passed, and then whenever a transaction is in flight. Spelling preserved from the original block, which installation projects read.
ActiveDevice INT Index of the device currently holding the bus, -1 when the bus is free.
Cursor INT Where the next selection pass starts. Watching this move is how you see fairness working.
Transactions UDINT Completed transactions since boot.
StepsExecuted UDINT Steps attempted; always at least Transactions. Divided by Transactions it is the batching ratio - how many of a device's register blocks came due in the same grant. A falling ratio on a faster bus is the bus keeping up with demand, not batching breaking: 2.7 with a 200 ms task, 1.4 with a 50 ms one.
StepFailures UDINT How many of those failed.
Watchdogs UDINT Steps abandoned because the transport never answered. Should stay 0.

Methods

DeviceCount — How many devices are registered on this bus. Exists so commissioning can walk the same list the scheduler serves, rather than being handed a second list to keep in step with this one.

GetDevice — The registered device at Index, counted from zero. Read-only access to the list, for anything that has to ask every device on the bus a question — commissioning is the one that does.

Parameter Type Default Description
Index INT Position in the registration list, from zero. Outside the registered range returns nothing rather than whatever is left in the array.

Init — Configures the bus controller, an overview of the parameters:

Parameter Type Default Description
Transport I_RS485_TRANSPORT The protocol implementation to drive.
StartupDelay TIME Wait this long after a cold start before talking to anything, so slaves that boot slower than the PLC are not written off as missing. Zero keeps the default.
SilenceTime TIME Quiet line between frames, both between the steps of one transaction and between transactions. Zero keeps the default.
StepTimeout TIME How long one step may take before the controller stops waiting for the transport and moves on. A watchdog on the transport, not on the slave — the slave's own reply timeout is shorter and lives in the transport. Zero keeps the default.

RegisterDevice — Registers an RS485 device function block with the bus controller. Call once at startup for each device on the bus. Returns FALSE if the bus already holds 32 devices.

Parameter Type Default Description
device I_RS485_DEVICE The RS485 device function block to register.

Code example

  • variables initiation:
I_RS485_TRANSPORT		: FB_RS485_TRANSPORT_RTU;
RS485BusController	: FB_RS485_BUSCONTROLLER;
  • Init calls (called once during startup, transport first):
I_RS485_TRANSPORT.Init(
	Port			:= SysCom.SYS_COM_PORTS.SYS_COMPORT1,
	Baudrate		:= 9600,
	Parity			:= SysCom.SYS_COM_PARITY.SYS_NOPARITY,
	StopBits		:= SysCom.SYS_COM_STOPBITS.SYS_ONESTOPBIT,
	ReplyTimeout	:= T#1S,
	GapTime			:= T#50MS
);

RS485BusController.Init(
	Transport		:= I_RS485_TRANSPORT,
	StartupDelay	:= T#5S,			(* let the slaves finish booting *)
	SilenceTime		:= T#50MS,			(* between frames, and between transactions *)
	StepTimeout		:= T#3S				(* watchdog on the transport *)
);
  • Adding a device to the bus (called once during startup):
RS485BusController.RegisterDevice(device := GVL_RS485.FB_RS485_EASTRON_SDM220_1);
  • Calling it cyclically. The controller drives the transport, so the transport instance itself needs no cyclic call:
RS485BusController();

Wiring note

The PFC200's onboard serial port must be put into RS485 mode once per controller with serialmode RS485 from the CODESYS PLC shell. No IEC code can do it, it survives a reboot, and until it is done the bus is silent with nothing to show for it. See Using Modbus RTU with the CODESYS 3S runtime.