Tony Duell ard at
Tue Nov 9 14:49:03 CST 2010

> De writes:
> >> I am looking for info on a Gandalf LDS120 modem, specifically the
> >> serial port pinout.
> > In the division of irrelevant to the original question, I thought these
> > things were line drivers, not modems.
> I always thought they got lumped into "short haul 4-wire modems".

It's difficult... The do 'modulate' and 'demodulate' so I guess they are 
modems. But they are certainly not normal dial-up modems.

> They do have "DCD" lights on the front. I seem to recall that it's just
> a light and doesn't actually assert any RS-232 pins. But they could just
> be differential line drivers probably with isolation.

I've found somenotes I made on it over 20 years ago. Even the 'scope 
photos of the line signals. Perhaps this will help...

To get inside, unplug it from the mains (!), then unscrew the 4 screws in 
the rubber feet. The cover then slides off backwards. The PCB is clearly 
visible (component-sdie up, there are transformers mounted under it.

The RS232 connector pins are  :
1 Prot Gnd
2 TxD
3 RxD
7 Sig Gnd
9 Rx Current Loop +ve
10 Rx Current Loop -ve

There is AFAIK no trasnmit current loop interface built-in. 

There are some jumper links on the PCB. : 

Cons/Ctl : Transmitter enable (Cons : oalways active, Ctl - controlled by 
RTS input)

2W/4W : 2 wire/4 wire line select. In 2W position, disable DCD on RTS

Term : Temrinate Rx Line with 150R

Line S/L : Line length select (set gain of Rx ampliifer

Tune : Adjust threshold of DCD comparator

Phase A/B (2 sets, must be set the same way) : swap inputs of RX 
flip-flop to correct for reversed line phase.

> 20+ years ago I'm sure I looked inside to see what's in there but I

It's all standard and common parts -- 324 Op-maps 339 comparators, etc.

> can't recall. I always thought they did some simplistic and almost certainl=
> y
> not Bell-standard FSK or PSK but
> that was just my impression, no actual evidence to back that up.

They're not FSK or anything similar.  What they do is send a pulse onto 
the line on each transition of TxD (a +ve going pulse for one transistion 
direction, a -ve going pulse for the other) and if there are no 
transitions on the line for about 1ms, then send another pulse of the 
same polarity. The Rx circuit detects the line pulses and drives an SR 

They're good to about 9600 buard.

> Did the 4-wire screws on the back have labels of "+" and "-"?


> That would be a point in favor of them being line drivers and not modems
> (although some simple modems were in fact phase-sensitive).

This unit is certianly pahse-sensitive. Swap the line wires round and 
you'll get inverted data (including start/stop bits) out.


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