Re: Substituting a modem for acoustic coupler

On 24 May 2006 06:54:57 -0700, brismith66@xxxxxxxxx put finger to
keyboard and composed:

The decision to run at 1350 was made long before my time. My
constraint is that I cannot force any changes to their infrastructure;
anything I have to do has to be "plug compatible." But I'm wondering
now whether they're going to have to finally bite the bullet and join
the 21st century. But the customer doesn't take news like that well
;-)

I never thought about the fact that you don't get a full cycle of a
1300Hz wave at 1350 baud.

Thanks for the pinouts of the Telxon. One of the things I'm wondering
about is whether I can take all the old Telxon Acoustic Couplers and
use them somehow with the new devices. The new Symbol device has a
cradle with an RJ-45 jack for a serial connection.

I would use a sinusoidal FSK generator IC such as the old XR2206 by
Exar:
http://www.exar.com/products/XR2206v103.pdf

I would drive the chip's input using a software routine that toggled
one of the RS232 pins, eg DTR or RTS. For example, a logic 1 could be
represented by a high on RTS and a logic 0 by a low. DTR could be the
enable output for data comms. The FSK output pin of the XR2206 could
drive a cheap audio amp such as the LM386 which would in turn drive a
cheap 8 ohm speaker.

If the acoustic couplers are isolated from the phone line, then you
may be able to place the additional circuitry within the unit. In this
case the "acoustic" circuitry will not be needed. Instead the FSK
output can directly drive the line transformer. This may not be
strictly legal, however, as your telecom authority may require all
devices to be tested and certified.

FWIW, I have the circuit diagram for an old 300 baud modem kit.
AFAICT, this can be modified to do exactly what you want at 1350 baud.
The circuit uses discrete components, ie no UART or CPU. This avoids
any "autobauding" issues which I suspect may be part of your problem.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
.