87 lines
6.8 KiB
Markdown
87 lines
6.8 KiB
Markdown
---
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title: 4G Modem — Bench Bridge & Cheap Alternatives to the Ardusimple 4G NTRIP Master
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tags: [project-drogheda, hardware, 4g, ntrip, modem, sourcing]
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project: Project Drogheda
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author: Helgen CTO
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date: 2026-07-08
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status: Decision support — bench (Block 9B) + production outlook
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---
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# 4G Modem — Bench Bridge & Cheap Alternatives
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> **Why this doc.** Block 9B (Modo B, watchdog in-line between the F9P and the 4G) needs the
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> 4G NTRIP Master out of the Pro's socket and on the protoboard — but its XBee pins are
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> **2.0 mm pitch** (not 1.27 mm; re-measure center-to-center: 10 pins span 18 mm) vs the
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> protoboard's 2.54 mm. Two questions answered here: (1) how to bridge it **this week**, and
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> (2) whether a **cheaper 4G module** can replace the €156 Ardusimple unit — bench now,
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> production later. Technician instructions live in the Guía v3.0 §9a–9c; this is the
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> engineering/sourcing record.
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## 1. Bridge options (bench, this week)
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| # | Option | Price (MXN) | Source / lead | Verdict |
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|---|--------|------------|----------------|---------|
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| A ★ | SparkFun **BOB-08276** breakout + 2× **PRT-08272** 2.0 mm sockets + 2.54 mm male headers | ≈$100 | [SANDOROBOTICS CDMX](https://sandorobotics.com.mx/producto/bob-08276/) — in stock, 1–2 days to QRO | **Buy today.** One soldering session; matches the Guía 9b procedure |
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| B | Generic "adaptador XBee" pre-assembled | $80–250 | Amazon MX / [MercadoLibre](https://listado.mercadolibre.com.mx/adaptador-xbee), 1–3 days | Fine fallback; verify DIP pins in photos |
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| C | XBee Arduino shield (HR0440) + Dupont F-F | $50–150 | SANDOROBOTICS, 1–2 days | Works as a fixed base next to the protoboard |
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| D | 2.0 mm F-F jumpers direct onto pins | $60–120 | AliExpress, 2–4 weeks | Last resort — fragile, slow to arrive |
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Power rule (already in the Guía): XBee **pin 1 VCC = 3.3 V, never 5 V**, ≥1.5 A peak →
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second LM2596/MP1584 set to 3.3 V, fed **from SAL** (so a stack power-cycle also reboots the
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modem — mirrors the PROD 3V3_MODEM rail), with 100 µF + 100 nF at the pin.
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Caution: SparkFun's *Explorer Regulated* uses a ~500 mA LDO — not enough for LTE bursts;
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use the bare breakout + our own buck.
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## 2. Cheap 4G module alternatives
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Telcel LTE bands to match: **B2/B4/B5/B7/B28/B66**. Key finding: **none of the cheap modules
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has a built-in NTRIP client** — the Ardusimple's real value is the on-module NTRIP client
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firmware (zero MCU work, proven in the field). Every alternative below buys hardware savings
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with firmware/integration cost.
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| Module | Price | Buy | Telcel bands | NTRIP path | Effort (1–5) |
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|--------|-------|-----|--------------|-----------|--------------|
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| **LILYGO T-A7670SA** (ESP32 + A7670SA) | ≈US$33 | [lilygo.cc](https://lilygo.cc/products/t-sim-a7670e) / AliExpress | ✔ (SA = LATAM, incl. B2/B4/B66) | ESP32 sketch: TinyGSM ([lewisxhe fork](https://github.com/lewisxhe/TinyGSM-fork)) + [Arduino NTRIP client](https://github.com/GLAY-AK2/NTRIP-client-for-Arduino); ESP32 does the client, streams RTCM out its UART | **2** — best bench experiment |
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| **A7670SA raw module** | **US$9.39** | [LCSC](https://www.lcsc.com/product-detail/2G-3G-4G-5G-Modules_SIMCom-Wireless-Solutions-A7670SA-LASC_C18548276.html) / MercadoLibre | ✔ | AT commands (CIPOPEN/CIPSEND) — NTRIP handshake + reconnect/watchdog logic on the Blackpill | **4** — the cost end-game for the hundreds-of-cabinets phase |
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| 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 |
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| SIM7670G (Cat-1 bis) | US$30–45 | [Waveshare ESP32-S3 combo](https://www.waveshare.com/esp32-s3-sim7670g-4g.htm) | ✔ | Same | 2–3 |
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| 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** |
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| **A7670E / SIM7600E** (Europe variants on MercadoLibre) | — | — | ✘ **no B2/B4/B66** | — | **Do not buy** |
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| Quectel EC25-AUX / EG25-G (mPCIe) | €44–60 | Sixfab/Waveshare | ✔ | AT/PPP + mPCIe carrier plumbing | 4 |
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| USR-G771 4G DTU | ≈US$43 | [usr.cn](https://en.usr.cn/RS232-RS485-To-4G-modem) / 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 |
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| **Digi XBee 3 Global LTE Cat 1** | ≈US$76 | [Digi-Key](https://www.digikey.com/en/product-highlight/d/digi-intl/xbee-3-global-lte-cat-1) | ✔ (global) | **MicroPython NTRIP client runs ON the module** (sockets API; [precedent](https://www.ardusimple.com/question/digi-xbee-3g-lte-micropython-ntrip-server-esp32-xbee-module/)) | 3 — the only true XBee-socket drop-in; keeps the EinarBoard footprint |
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Reference: Ardusimple's own WiFi NTRIP Master firmware is open source
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([nebkat/esp32-xbee](https://github.com/nebkat/esp32-xbee), ESP-IDF NTRIP client/caster) —
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an ESP32-as-NTRIP-bridge is a solved problem; 4G needs the WiFi transport swapped for
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TinyGSM/PPP. Industrial NTRIP DTUs (Tersus TR600, Hi-Target) exist but run US$300–800 —
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*more* than the Ardusimple, not less.
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For scale reference: **Ardusimple 4G NTRIP Master = €156** (≈MX$3,250), in stock, DHL 2–5
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days, **€112 at ≥50 units** ([product page](https://www.ardusimple.com/product/4g-ntrip-master/)).
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LTE Cat 4, NA variant covers Telcel B2/B4/B5.
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## 3. Recommendation
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1. **This week (bench, keeps 17-Jul comparison on track):** order Option A (~MX$100,
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SANDOROBOTICS) and breadboard the 4G NTRIP Master we already own per Guía v3.0 §9a–9c.
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Zero firmware risk.
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2. **Parallel experiment (~US$35):** one **LILYGO T-A7670SA** + TinyGSM NTRIP sketch
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(~1 day of firmware). If it holds RTK for a week on the bench, it becomes the certified
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cost-reduction candidate to qualify properly.
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3. **30 units / August:** stay on the Ardusimple (€112 @ ≥50). It is the only zero-firmware,
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field-proven option and the timeline doesn't absorb a bearer swap. Invariant #3
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(corrections ride the 4G; recover by power-cycling) is already engineered around it.
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4. **Hundreds-of-cabinets phase:** A7670SA soldered onto the EinarBoard successor
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(~16× cheaper bearer) **budgeting real firmware** (NTRIP-over-AT + reconnect watchdog —
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the part Ardusimple already does well) + carrier homologation; or the **Digi XBee 3 Cat 1**
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(US$76) as the middle path that drops into the existing XBee socket with the NTRIP client
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in MicroPython on-module. Decide after the LILYGO experiment.
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## 4. Cross-references
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- Technician instructions (Spanish): `Drogheda_Guia-Conexion-Protoboard_ES_v3.0.pdf` §9a–9c.
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- PROD context: 3V3_MODEM rail (Block K) + K1 NC bypass — `watchdog-electrical-design-state.md`.
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- Invariants touched: #1 (power-off safe), #2/#3 (never vanish; corrections ride the 4G —
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any replacement bearer must keep the power-cycle recovery path).
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