Hi,

> Does anyone have something to say about single chip modems? I'm considering
> putting a 2400bps modem chip on one of my PIC data acquisition boards but
> would like some comments/suggestions/flames.

I will assume you want to use a Telephone line modem.
Radio modems have similar tradeoffs though.

I have in the past found that the run of the mill pocket modem
that has a standard serial port works well.  I have used these
in remote applications with Basic Stamps without trouble.  Sending
the init string and such is all it takes with a serial port and you
can ignoore all the other pins by strapping them high.  Wait for the
DCD line to go high or the CONNECT string wait a few seconds and then
strat sending data.  Just to receive calls is easier, just wait for
your protocol packets from the host and let it do the call establishment
and hanging up.

Prices should be in the US$75 to US$200 range for a 2400 to 14400
modem  (if you have the space a full size external modem will be a bit
cheaper)  and you get all the DAA (data access arrangemet) isolation,
relays, ring detectors, &c.  And you can store the phone number in the
modem and change it without having to change your PIC/STAMP code.

Internal card modems are even cheaper and all you need is the idea
from the PIC compertition handbook where one of the entrants uses a
PIC to generate ISA bus signals (8 data, 3 address and IOR, IOW, RST)
with all the other signals fixed to the nominal levels, this will
require you to program the 8250/16450/16550 UART on the card directly
but this is quite simple with code in BIOS dissasemblies, IBM tech-refs,
PC interfacing books and also in the Startech UART data book (very
cheap 16550 UARTS, won't work on fast PCs though > AT bus speeds).

On the other hand using a modem chip requires all the analog hang on stuff
and you wont likely get FCC or Bell (Telco) approval unless you are
prepared to go through the type aproval schlep which will cost
quite a lot.

Volume will dictate.

Cheers
--
Kalle Pihlajasaari     kalle@data.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751