[TenTec] OMNI VI Plus, MMTTY, and FSK

[Ed]

KA5M at aol.com wrote:
> Ed, I'm just getting started on FSK RTTY, and want to understand the basics 
> of the radio to computer interface.  How does a signal from TXD (pin 3) of a 
> DB9 serial port connector, through a 1K resistor, to Mark/Space on the rear of 
> the OMNI VI Plus, put the transceiver into FSK transmit?  My limited 
> understanding is that all Mark/Space input on the rear does is control which tone, Mark 
> or Space, is being sent.  What am I missing?
> 
> Also, which schematic are you referring to?
> 
> 73,
> Marsh, KA5M

Marsh, since you originated this thread, I'll reply to you and the 
others that have contributed to this thread can follow along if they 
wish. I have been using FSK not only with the Omni VI, but also the Omni 
V for the past 5-6 years using the setup I described to you.

Please refer to page 3-2 and 3-3 in your Omni VI manual starting with 
3-1.07 through 3-1.08. This will give you the basic idea of the 
differences between FSK and AFSK as it pertains to the Omni. Next refer 
to page 4-43 and 4-44 for the explanation of the FSK circuit. The 
schematic is on page 4-45, look in the lower left side for the 
Mark/Space jack, you will see that it goes to a NPN then on to a PNP 
then the FSK generator.

Now for those that do not have an Omni VI manual, from page 4-44 I'll 
quote what TT says about FSK keying

"Note: FSK MARK input voltage can be in the range of 0 to -15 Vdc. FSK 
SPACE input voltage should be in the range of +2.5 to +15 Vdc for proper 
operation."

If you take the time to measure the voltage on the typical TXD line of a 
serial port you will notice it will swing betweeen +15 and -15 Vdc 
depending on if a Mark or Space is being sent by your software. In other 
words, if the software sends a Mark it is hard coded in the software to 
shift the TXD line low and visa versa for Space, it will go high or a + 
15 volts. The transistors in the Omni determine whether a Mark or Space 
is sent based on whether they detect a + or - voltage and act accordingly.

All I can say to the others saying a PTT is necessary, you do if you are 
using a rig that uses audio derived FSK (Icom, Kenwood, Yaesu) come to 
mind. The Omni V and VI use a true FSK circuit that relies on a keyed 
plus or minus voltage. Audio derived FSK relies on high, low tones to 
send the FSK that is generated by the software. Basically the software 
is generating the FSK, not the rig. ....73 HTH

Ed W3NR