[HomeBrew] An RTS-DTS Reed relay question

[Philip Atchley]

Hi,

I'm in the process of building a "digital interface" so that I can use my Icom 
IC-751 in the digi and CW modes with the computer.  I've decided to an 
entirely passive design, using 600/600 Ohm isolation transformers for the 
RX/TX audio lines (Xformers pulled from scrapped phone gear).

For the key lines I've decided to use some 12 Volt reed relays I have laying 
around (CHEAP is the name of the game here).  I've used opto couplers in the 
past, but ran into one case where I had some stray RF, so once I keyed the rig 
(CW mode) it stayed keyed 8^( I figure with the isolation transformers and 
reed relays I won't have any ground loops to create those ugly 60 Hz lines in 
the waterfall display (I've been playing with receiving RTTY and DGPS on the 
radio/computer and had to use a transformer for isolation).

I've copied some interface circuits off the web and have chosen the "best" 
points of each for my own project.  Transistor switching is out because I'd 
still have a ground connection between radio and computer, I want total 
isolation.

My Question is this. Just how much current can I safely draw off the RTS/DTS 
lines on the RS-232 port (desktop, NOT laptop).  The relays I have are 500 Ohm 
coils, one for PTT (RTS line) and one for the CW Key line (DTS in CW MODE). 
(I've also considered using just one relay on the DTS line for both PTT for 
DIGI modes and Keyline in CW mode and using QSK to switch the TX. That 
requires an added toggle switch to switch the relay between the two radio 
ports.

73 de Phil,  KO6BB
http://www.geocities.com/ko6bb/
Merced, Central California, 37.3N  120.48W  CM97sh