Modem refuses to dial for a micro controller

[mordax]

I want a micro controller to communicate via a ds276 chip with a Trust MD1200 modem. This works fine (e.g. with ATI.. and ATE.. commands) but the moment it encounters a dial command within a command string it returns “error”. However, if I talk to it via hyperterminal it dials ok. I have tried baud rates between 300 and 19200, no difference. My voltage levels at sout of the ds276 (tx) are: mark = -7.5 V, space = 5 or 12 V (tried both)and the pulses look good. Example
(Monitored via a breaker box by hyperterminal):
ATI4e0tH0Ti3E1i2D4464593 /*echo from the terminal*/
DSP Ver3.00 CheckSum=668C /*response to I4*/
TP560 Data/Fax/Voice 56K Modem /* response to i3*/
TEST OK! /* response to i2*/
ERROR. /* response to D, lower case d, same thing*/
HELP!

 

[mordax]

Ok, I can now answer my own question. My problem was: I naivly assumed that Tx,Rx and ground is sufficient. It is not, the Modem wants to see "Data Terminal ready" and "request to send" on. The confusing thing was the strange behaviour of my particular modem: doing everything without these signals except dailing.