Sending ASCII control strings using C8P modem port to RS-232C device

[Mark]

I assume pin 7 of Heathkit U4 optoisolator with the FTDI cable with BREAK on is much lower? < 2.4v? If not, what is it?

If so then it seems the OPTO isn't turning on enough with the OSI's RS-232 voltages.

In that case I think R11 needs to be ~2K ohms or so to work with the OSI.

You can try putting a 6.8K resistor in parallel with R11 to drop the series resistance to ~2200 ohms, which should be enough to trigger the opto with the OSI. If not then try 4.7K which drops the resistance a little further. (Trying to get it low enough to trigger with the OSI without sending too much current when using FTDI cable.)

Or you could fix the OSI with an RS-232 driver chip like the MAX-232 or wire in one of the prebuilt TTL to RS-232 modules that handles at least Rx and Tx.

Or mod the output driver of the OSI with a higher voltage source if available (12v?). (Emitter of Q2 to 12V through resistor )?

Anyway, I think you've located the problem! Good Luck!

[lowrybt1]

Mark and Danny,

Much appreciated for all the sleuthing. I'm going to go with the TTL-RS232 approach. Tom

[lowrybt1]

I found an old RS232 powered line extender on ebay. It takes the OSI modem port voltage range of 0-5VDC and changes the output range to something like -9 to 5VDC. The extender appears to generate and/ or seek signaling on the CTS, RTS, DCD and DTR lines. For reasons I've not yet figured out I needed to tie the DTR line to the RTS to get the modem port to send data on its TX line (pin 2 of the modem port). In the end, I'm now able to successfully signal the Heathkit X10 controller. Am well on my way to revising the old BSR X10 software authored by Fred S to use the modem port and Heathkit device to schedule the use of X10 home control devices.

While waiting for the RS232 extender to arrive, I went to work on using my 567 board to control X10 home control devices by dialing and sending DTMF commands to an old BSR X10 phone-line connected console. Modified the CA-15 phone demo software to make this work. Revised (stripped down) the ring/busy detect subroutine to cope with the X10 console's 7-ring wait before answering calls and its 3-beep okay-to-send-commands signal. And added a subroutine for sending the necessary tones via the 567 board to turn x10 devices on/off over the phone. (Using a line simulator) Fun project and a Rube Goldbergian approach to using an OSI C8P to interface with home control devices.

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