7. Z21 connection (RB1110 and dual-protocol centrals)

For some command stations BigFred connects over the Z21 LAN protocol (UDP) instead of LocoNet serial via the Uhlenbrock 63120. The Pi 5 host stack (§3) is the same; you omit the Uhlenbrock 63120 when using Z21.

7.1 When to use Z21 vs LocoNet serial

Command station	Recommended BigFred connection	This document set
Digikeijs DR5000	`loconet_serial` + Uhlenbrock 63120 on LocoNet-T	§2–§6 (LocoNet path)
RailBOX RB1110 / RB1110-Mini	`z21` over LAN/WiFi	This chapter
Other centrals with both Z21 and LocoNet	Prefer `z21` if the vendor documents it; LocoNet via Uhlenbrock 63120 is possible	Best-effort — no guarantee (§7.5)

The RB1110 exposes LocoNet on RJ12 for handhelds and modules, but RailBOX documents Z21® as the primary PC/tablet integration path. BigFred follows that: use z21, not loconet_serial + Uhlenbrock 63120, for RB1110 hubs.

7.2 Data path (RB1110)

Browsers ──HTTP/WS──► loco-server (Pi 5)
                          │
                          ▼
                     dcc-bus ──► UDP Z21 (port 21105)
                          │
                     RB1110 (WiFi or Ethernet on club LAN)
                          │
                          └──► DCC ──► layout

Web → track: loco.setSpeed → Z21 LAN_X_SET_LOCO_* → RB1110 → DCC.
Track → web: Z21 LAN_X_LOCO_INFO push (after broadcast flags) → z21.go → subscribed clients.

Driver reference: docs/bigfred/protos/z21.md, 16-dcc-bus/09-external-state-observation.md.

7.3 Network setup

Requirement	Detail
Pi and RB1110 on the same LAN	Prefer Ethernet on the Pi; WiFi on the central is fine if stable
Fixed IP for RB1110	DHCP reservation or static IP in the club runbook
UDP port	21105 (default Z21); firewall must allow Pi → central UDP
No Uhlenbrock 63120	Not used on this path

Configure the RB1110 network (SSID, IP) with RailBOX tools per the manufacturer manual before pointing BigFred at it.

7.4 BigFred catalogue entry

Field	Value
Kind	`z21`
URI	`udp://192.168.1.50:21105` (host = RB1110 IP; port optional)
Speed steps	`128` (match decoder / central settings)

Parser (driver.go): udp://host:port or host:port; port defaults to 21105.

Example admin name: RB1110 (Z21).

dcc-bus starts with --station-kind z21 and dials UDP; there is no serial device to udev-pin.

Capabilities over Z21

See z21.go — generally broader than basic LocoNet in BigFred (e.g. more functions, CV access). Hub semantics (many drivers, takeover, one daemon per command station) are unchanged.

7.5 Dual-protocol centrals (best-effort)

Centrals that advertise both Z21 LAN and LocoNet (e.g. some Roco Z21 variants with LocoNet ports, club loaned hardware, future models) may work with BigFred using either:

z21 — udp://…:21105, or
loconet_serial — Uhlenbrock 63120 on the LocoNet throttle port (§4).

There is no official guarantee until the combination is tested on your layout. Prefer the connection mode the manufacturer documents for PC control; for RB1110 that is explicitly Z21.

Report gaps (missing push, slot limits, accessory commands) as BigFred issues with the central model and firmware version noted.

7.6 Bring-up checklist (RB1110 / Z21)

[ ] RB1110 powered; track power as required; IP reachable from Pi (ping).
[ ] nc -u -z or socat smoke test optional; then BigFred catalogue z21 row.
[ ] dcc-bus log shows z21 command station: UDP socket open.
[ ] Driver session: setSpeed moves a loco.
[ ] Physical throttle change reflected in UI (Z21 broadcast / subscribe path).
[ ] Second browser driver on another loco.

Troubleshooting: wrong IP, VLAN isolation, firewall blocking UDP 21105, RB1110 WiFi sleep, or another app (mobile RailBOX app) holding the Z21 session — only one writer per command station (§5.6).

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