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| title | tags | project | author | date | status | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 4G Modem — Bench Bridge & Cheap Alternatives to the Ardusimple 4G NTRIP Master |
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Project Drogheda | Helgen CTO | 2026-07-08 | Decision support — bench (Block 9B) + production outlook |
4G Modem — Bench Bridge & Cheap Alternatives
Why this doc. Block 9B (Modo B, watchdog in-line between the F9P and the 4G) needs the 4G NTRIP Master out of the Pro's socket and on the protoboard — but its XBee pins are 2.0 mm pitch (not 1.27 mm; re-measure center-to-center: 10 pins span 18 mm) vs the protoboard's 2.54 mm. Two questions answered here: (1) how to bridge it this week, and (2) whether a cheaper 4G module can replace the €156 Ardusimple unit — bench now, production later. Technician instructions live in the Guía v3.0 §9a–9c; this is the engineering/sourcing record.
1. Bridge options (bench, this week)
| # | Option | Price (MXN) | Source / lead | Verdict |
|---|---|---|---|---|
| A ★ | SparkFun BOB-08276 breakout + 2× PRT-08272 2.0 mm sockets + 2.54 mm male headers | ≈$100 | SANDOROBOTICS CDMX — in stock, 1–2 days to QRO | Buy today. One soldering session; matches the Guía 9b procedure |
| B | Generic "adaptador XBee" pre-assembled | $80–250 | Amazon MX / MercadoLibre, 1–3 days | Fine fallback; verify DIP pins in photos |
| C | XBee Arduino shield (HR0440) + Dupont F-F | $50–150 | SANDOROBOTICS, 1–2 days | Works as a fixed base next to the protoboard |
| D | 2.0 mm F-F jumpers direct onto pins | $60–120 | AliExpress, 2–4 weeks | Last resort — fragile, slow to arrive |
Power rule (already in the Guía): XBee pin 1 VCC = 3.3 V, never 5 V, ≥1.5 A peak → second LM2596/MP1584 set to 3.3 V, fed from SAL (so a stack power-cycle also reboots the modem — mirrors the PROD 3V3_MODEM rail), with 100 µF + 100 nF at the pin. Caution: SparkFun's Explorer Regulated uses a ~500 mA LDO — not enough for LTE bursts; use the bare breakout + our own buck.
2. Cheap 4G module alternatives
Telcel LTE bands to match: B2/B4/B5/B7/B28/B66. Key finding: none of the cheap modules has a built-in NTRIP client — the Ardusimple's real value is the on-module NTRIP client firmware (zero MCU work, proven in the field). Every alternative below buys hardware savings with firmware/integration cost.
| Module | Price | Buy | Telcel bands | NTRIP path | Effort (1–5) |
|---|---|---|---|---|---|
| LILYGO T-A7670SA (ESP32 + A7670SA) | ≈US$33 | lilygo.cc / AliExpress | ✔ (SA = LATAM, incl. B2/B4/B66) | ESP32 sketch: TinyGSM (lewisxhe fork) + Arduino NTRIP client; ESP32 does the client, streams RTCM out its UART | 2 — best bench experiment |
| A7670SA raw module | US$9.39 | LCSC / MercadoLibre | ✔ | AT commands (CIPOPEN/CIPSEND) — NTRIP handshake + reconnect/watchdog logic on the Blackpill | 4 — the cost end-game for the hundreds-of-cabinets phase |
| SIM7600SA-H (Waveshare HAT / LILYGO T-SIM7600) | US$40–77 | Waveshare / LILYGO | ✔ | Same AT/ESP32 paths | 2–4 — Cat-4 is overkill, no cost win vs A7670SA |
| SIM7670G (Cat-1 bis) | US$30–45 | Waveshare ESP32-S3 combo | ✔ | Same | 2–3 |
| SIM7000G (Cat-M1/NB-IoT) | US$40–69 | Botletics | Bands ✔ but LTE-M is a Telcel enterprise product — SIM provisioning risk | AT/TCP | 4 — avoid |
| A7670E / SIM7600E (Europe variants on MercadoLibre) | — | — | ✘ no B2/B4/B66 | — | Do not buy |
| Quectel EC25-AUX / EG25-G (mPCIe) | €44–60 | Sixfab/Waveshare | ✔ | AT/PPP + mPCIe carrier plumbing | 4 |
| USR-G771 4G DTU | ≈US$43 | usr.cn / Amazon | verify variant (-E = Europe) | Transparent TCP + "registration packet" carrying the NTRIP GET — no MCU firmware | 2–3 — closest cheap drop-in if a MX-band variant exists |
| Digi XBee 3 Global LTE Cat 1 | ≈US$76 | Digi-Key | ✔ (global) | MicroPython NTRIP client runs ON the module (sockets API; precedent) | 3 — the only true XBee-socket drop-in; keeps the EinarBoard footprint |
Reference: Ardusimple's own WiFi NTRIP Master firmware is open source (nebkat/esp32-xbee, ESP-IDF NTRIP client/caster) — an ESP32-as-NTRIP-bridge is a solved problem; 4G needs the WiFi transport swapped for TinyGSM/PPP. Industrial NTRIP DTUs (Tersus TR600, Hi-Target) exist but run US$300–800 — more than the Ardusimple, not less.
For scale reference: Ardusimple 4G NTRIP Master = €156 (≈MX$3,250), in stock, DHL 2–5 days, €112 at ≥50 units (product page). LTE Cat 4, NA variant covers Telcel B2/B4/B5.
3. Recommendation
- This week (bench, keeps 17-Jul comparison on track): order Option A (~MX$100, SANDOROBOTICS) and breadboard the 4G NTRIP Master we already own per Guía v3.0 §9a–9c. Zero firmware risk.
- Parallel experiment (~US$35): one LILYGO T-A7670SA + TinyGSM NTRIP sketch (~1 day of firmware). If it holds RTK for a week on the bench, it becomes the certified cost-reduction candidate to qualify properly.
- 30 units / August: stay on the Ardusimple (€112 @ ≥50). It is the only zero-firmware, field-proven option and the timeline doesn't absorb a bearer swap. Invariant #3 (corrections ride the 4G; recover by power-cycling) is already engineered around it.
- Hundreds-of-cabinets phase: A7670SA soldered onto the EinarBoard successor (~16× cheaper bearer) budgeting real firmware (NTRIP-over-AT + reconnect watchdog — the part Ardusimple already does well) + carrier homologation; or the Digi XBee 3 Cat 1 (US$76) as the middle path that drops into the existing XBee socket with the NTRIP client in MicroPython on-module. Decide after the LILYGO experiment.
4. Cross-references
- Technician instructions (Spanish):
Drogheda_Guia-Conexion-Protoboard_ES_v3.0.pdf§9a–9c. - PROD context: 3V3_MODEM rail (Block K) + K1 NC bypass —
watchdog-electrical-design-state.md. - Invariants touched: #1 (power-off safe), #2/#3 (never vanish; corrections ride the 4G — any replacement bearer must keep the power-cycle recovery path).