Tech Flashback: Netcomm SmartModem 2400SA

Another one of my “vintage” acquisitions thanks to Kafoopsy from OCAU (again) – I have my hands on a Netcomm SmartModem 2400SA.

Netcomm SmartModem 2400SA Front

It might need some retro-bright, but this is an early-ish SmartModem. It’s not the earliest – after all, this is a 2400bps modem, and things started at 300bps. But it is still very much a classic recognizable appearance. The body is made of a metal shell with plastic ends – “built like a tank” compared to modern telephony devices.

Netcomm SmartModem 2400SA Rear

This modem straddled a time where SmartModems were new and novel. As a result, there is a set of DIP switches at the rear for configuration that allows you to turn it into a “dumb modem”. It also allows for override of CTS/CD signals in case of compatibility issues.

A SmartModem differed from a dumb modem that it didn’t just directly convert the serial port’s voltage levels into tone frequencies (direct FSK). It also does higher-order modulations – and may involve some digital signal processing. More importantly, it is controlled by the AT command set – allowing a computer to take control of the phone line, and dial without human intervention. It also involves abilities such as Asynchronous operation – meaning that the serial port can operate at a speed different to that of the negotiated modem connection.

This particular SmartModem offered several modulation types for each speed – so part of the smarts lie in the auto-ranging ability which allows for the modem (in most cases) to successively attempt modulations from high to low and automatically establish the best possible connection given the conditions and combinations of modems.

Later modems would offer compression, link management, error correction, voice and fax capabilities.

Netcomm SmartModem 2400SA Label

Netcomm is a very Australian brand – their success started in SmartModems. The gear was considered quality and was quite expensive (I’d say about $500AU in 1992 dollars judging from this).

Netcomm SmartModem 2400SA Power Adapter

The power adapter is a pretty standard 9V AC adapter. A modem wasn’t complete on its own. This modem came with software called “The Netcomm Program” on three 5.25″ floppies (DS/DD, IBM format). The disks were still readable with the Kryoflux, I might explore the software (basically a terminal emulator) later.

Netcomm Program Disk Front Netcomm Program Disk Rear

Also included was extensive documentation for the modem in the form of a wire-spiral bound illustrated manual with coloured section divider pages (click any of the images for larger)

Netcomm Modem Manual

Netcomm Modem Manual Page

A great effort – although you might expect it at that price for the modem. Interestingly, looking at the rear of the manual for the contact details, it makes reference to several defunct technologies – Keylink, Viatel and Telex. I was too young to have used any of these services – but it’s an interesting lead to follow. I was aware of BBSes, Fidonet and Telex but Keylink and Viatel are new to me.

Anyway, there was an even thicker wire-spiral bound manual for the software …

Netcomm Program Manual

… registration forms …

Netcomm Registration Card Front Netcomm Registration Card Rear

… program errata and TSR advice (implying this modem was shipped in 1991 at the earliest) …

Netcomm Program Errata Netcomm TSR Warning

… and a quick reference guide to AT commands.

Netcomm AT Guide Front Netcomm AT Guide Rear

Opening the rear of the modem and removing the power-regulator clamp screw (using the case as a heatsink) reveals a very bowed circuit board. Yikes! All those years of heat, humidity and gravity must have taken its toll.

Netcomm SmartModem 2400SA Open

Removing the PCB reveals something quite similar to the early Netcomm I received (that was non-functional). A double sided PCB in light green.

Netcomm SmartModem 2400SA

The phone line is switched by a Fujitsu branded relay. The modem itself is made completely with through-hole components – involving large round flat-ended indicator LEDs, a speaker glued to the board, many CMOS DIP chips, a EPROM from Motorola with the firmware and a Rockwell chipset. I was surprised to find that Rockwell had been doing modem chipsets for *this* long. Wow! Interesting is the unpopulated DIP slot in the middle – I wonder what that’s used for.

The Rockwell chipset there is also interesting because it’s a QIL chip as opposed to a DIL chip. This form of leg-arrangement is very rarely seen.

The board says Made in Australia on the edge – along with NetComm (Aust.) P/L SMART MODEM 1234 NC363-1. These were apparently used in alarm boards as well.

Netcomm Smartmodem 2400SA

The underside of the board doesn’t hold many surprises. The linear regulator is mounted upside down to allow for it to make contact with the case as a heatsink.

I plugged it in and turned it on. It lives! Using a modern computer with a USB to RS-232 (with level shifter, not the cheapy TTL only ones), it was possible to get it to work.


As I don’t have a land-line, I just used my (optimized) VoIP service to make several test calls. By intercepting the packets through a transparent bridge, we can get high quality (relatively speaking) recordings of the modem negotiating with a V.90 partner.


The left channel is the answering modem, the right channel is the local modem (i.e. the Netcomm). By using the ATBx command, we could force certain modulations. The audio includes handshaking and the connection with the data above traversing (mostly). It can be quite annoying to some due to continuous tones being used – but that’s how it was in the early days with one frequency shift keying “pair” used in each direction (or phase shift keying in the higher rates prior to QAM/Trellis).

In the last case, 2400bps Bell modulation was selected, but as Bell modulation is forbidden in Australia (from memory), the connection fails virtually immediately.

But it’s rather exciting to think that a modem from 1991 (22 years ago) only did 2400bps and still works. 2400bps is just 240 bytes per second (accounting for 1 start bit + 8 data bits + 1 stop bit). At this rate, a 5MiB MP3 file would take … *crunches* … about 6 hours and 4 minutes to download!


About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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