For a single conveyor cell polling an RTU/PLC into a SCADA master, the modern default is a 4G LTE Cat 1 or Cat 4 industrial modem (also called DTU) with an RS-485 port, 7-30 VDC input, and a transparent serial-to-IP tunnel; LTE-M is the right call only when the cell is solar- or battery-powered and the SCADA polling interval stretches past one minute [S1].
The job of the modem in this stack is narrow: take Modbus RTU, DNP3 serial, or DF1 off a PLC, push it over a cellular bearer to a carrier APN, and present a TCP socket to the SCADA master at the other end [S1]. Cable runs of 15 m on RS-232 or up to 1,200 m on RS-485 define the field topology, and a tower- or panel-mounted LTE antenna on LMR-200, LMR-400, or RG-58 coax closes the radio link [S1].
Matching the Cellular Bearer to Polling Profile
The first selection gate is the SCADA polling cadence versus the radio technology, and a clean rule of thumb splits the candidates cleanly [S1]. If the PLC is polled faster than once per second and the cell also pushes firmware images to remote I/O, LTE Cat 4 is the only option that survives the upstream without queueing; if the cell is read once a minute and runs on solar or battery, LTE-M cuts idle power by an order of magnitude compared to Cat 4; if the device sends payloads of 100 bytes or less once an hour from a meter, NB-IoT is the lowest-energy path [S1].
Cat 1 sits between those extremes and is the realistic default for a single conveyor cell: it carries several kB of poll-and-response traffic per cycle, falls back to 3G/2G on most operator networks, and is priced closer to LTE-M modules than to full Cat 4 hardware [S3]. For video-surveillance or AGV-fleet cells where the same modem also carries HD camera streams or 100+ vehicle telemetry feeds, Cat 4 is mandatory because the downlink bandwidth of Cat 1 is too thin for that mix [S3].
Serial Interface, AT Command Set, and Default Parameters
Every industrial modem in this class exposes a Hayes-compatible AT command set over a DB9 RS-232 port, and the factory default on most units is 115200 baud, 8 data bits, no parity, 1 stop bit, with hardware flow control enabled [S1]. That is the configuration a service laptop should expect on first contact, and it is also the configuration most Modbus-RTU-to-TCP gateways expect to negotiate in transparent mode [S1].
RS-485 is the field-bus pick for any conveyor cell longer than a few metres: the standard rate is 10 Mbit/s but the realistic bus length at 115200 baud is the cited 1,200 m, which lets a single modem daisy-chain multiple VFDs, weigh scales, and photo-eyes along one trunk [S1][S2]. Devices such as the rugged D80-series DTU expose RS-232, RS-485, RS-422, and TTL on the same board precisely so the same modem can sit on a Modbus RTU trunk, a legacy RS-422 encoder, or a 3.3 V TTL sensor cluster without an external converter [S2].
Hazardous-Area Certification When the Conveyor Crosses a Flammable Zone
If the conveyor cell runs through a Class I Division 2 zone as defined by NFPA 70 Article 500 - flammable vapours normally confined, but able to escape under abnormal conditions - the modem itself must carry the cULus, CSA, ATEX, or IECEx mark on the agency certificate, not just on the datasheet [S1]. The four NEC gas groups A (acetylene), B (hydrogen), C (ethylene), and D (propane, gasoline) span ignition energies from 0.017 mJ to 0.240 mJ and autoignitions from 305 °C to 571 °C, and a mark reading Class I Div 2 Groups A,B,C,D with a T4 (≤135 °C) or T5 (≤100 °C) temperature code covers virtually every refinery, wellhead, and tank-farm deployment [S1].
Two practical points often missed in procurement: a Class I Div 2 rating does not require an explosion-proof enclosure, but the modem still goes inside a NEMA 4 or 4X panel for environmental protection, and the rating applies to the modem case and connectors, not to the panel around it [S1]. The agency file is a UL CCN of the NRAG family for the U.S., and the certificate number should be requested before purchase because the hazardous-location mark is on the certificate, not the datasheet [S1].
Power, Antenna, and Carrier-Side Wiring
Input voltage is the second hard gate: industrial modems in this class accept 7-30 VDC, which covers 12 V and 24 V DC bus rails common on conveyor control panels, and lets the same modem run on a 12 V solar battery with a DC-DC step-up to the panel bus [S1][S2]. A keepalive or scheduled-polling pattern is the standard power-saving mode; always-on is for cells where the SCADA master expects an unsolicited report inside a 30-60 s heartbeat window [S1].
On the radio side, antenna choice is not a detail: LMR-200 is fine for runs under ~3 m, LMR-400 is the workhorse up to ~15 m with low loss at 700-2700 MHz, and RG-58 is acceptable only for very short jumpers and will eat several dB on anything longer [S1]. The carrier APN terminates at a VPN concentrator on the SCADA master side, which is the only way to keep Modbus TCP traffic out of the public Internet on a brownfield plant [S1].
Criteria-Based Comparison of the Main Modem Variants
Lining the four common options up against the criteria that actually drive a conveyor-cell decision - bandwidth, power, latency, and cost - shows where each one wins: [S1]
- LTE Cat 4: 150 Mbit/s down / 50 Mbit/s up class, 5-10 W under load, sub-100 ms latency on idle, $80-$150 module cost; right when the cell also streams video or carries 100+ AGVs on a shared APN [S1][S3].
- LTE Cat 1: 10 Mbit/s down / 5 Mbit/s up, 2-4 W under load, $25-$60 module cost; the right default for a single conveyor cell polling a PLC every second or two [S3].
- LTE-M (eMTC): ~300 kbit/s up / ~375 kbit/s down, 0.1-0.5 W in PSM/eDRX, $15-$30 module cost; the right pick when the cell is solar- or battery-powered and the polling interval is 60 s or longer [S1].
- NB-IoT: ~26 kbit/s up / ~62 kbit/s down, sub-mW in PSM, $8-$15 module cost; the right pick only for ≤100-byte hourly meter reads, not for conveyor SCADA where the round-trip will be too slow for any motion-control feedback [S1].
Who Should Not Pick the Mainstream Cat 1 Option
Engineers specifying Cat 1 for a solar-powered remote conveyor cell will be disappointed: Cat 1 idle power is roughly an order of magnitude higher than LTE-M, and the duty cycle required to keep the radio registered on a 1-min polling interval will flatten a 20 Ah battery in days rather than weeks [S1]. Likewise, picking NB-IoT for a fast-moving cell with VFD fault callbacks under one second will produce timeouts, because NB-IoT's uplink scheduling and PSM wake-up add seconds of latency to every transaction [S1].
Engineers specifying RS-232 for a conveyor line longer than 15 m will hit the cable-length limit before they reach the second drive, and any 24 VDC rail that dips below 7 V during a motor start will brown-out the modem and drop the SCADA link [S1][S2]. For cells near paint booths, wash-down areas, or chemical dosing skids, the IP65 enclosure and conformal coating option on the modem is non-negotiable, and the industrial buzzer used for the local alarm must carry the same enclosure rating if it sits on the same panel [S1].
Standards, Sourcing, and Procurement Checks
The standards backbone for a conveyor-cell modem is straightforward: NFPA 70 Article 500 for the U.S. hazardous-location classification, the cULus NRAG CCN or equivalent ATEX/IECEx certificate for the modem itself, and NEMA 4 or 4X for the surrounding panel [S1]. For European-sourced equipment, the ATEX 2014/34/EU equipment directive and the IECEx scheme cover the same zone, and the temperature code must match the worst-case gas autoignition minus a 20-30 % safety margin [S1].
On the SCADA side, the load cell and weigh scale on a check-weigher conveyor typically expose Modbus RTU over RS-485, which means the modem and the scale share the same trunk and the same polling register map; the load cell module that aggregates several scales will sit on the same bus and present one Modbus ID to the SCADA master through the modem. Sourcing-wise, request the agency certificate number, the AT command reference, the firmware revision policy, and a confirmation that the carrier bands match the local LTE deployment before signing the PO [S1][S2].
The shortlist logic for a single conveyor cell is therefore: default to Cat 1 on RS-485 at 115200 8N1 with a T4-rated cULus modem in a NEMA 4X panel; step up to Cat 4 only if the cell also carries video or a large AGV fleet; step down to LTE-M only if the cell is solar-powered and polled once a minute or slower; avoid NB-IoT entirely for conveyor SCADA. Watch the AT command reference for hardware-flow-control defaults, the carrier-band lock against the local LTE deployment, and the keepalive/PSM behaviour against the SCADA master's dead-band timer; those three items are where most field failures actually start.
For related coverage, see Industrial Buzzer Selection Criteria for Packaging Line Retrofit.