Bit of an update from this weekend.
THe MAX232 Module idea ended up with some magic smoke
I do not know if this is just poor modules from China, or an overenthusiastic R and D engineer.
I changed plans and decided to try to make what I have work.
So, based on an earlier Double Current Card (V1.4) I started some modifications earlier today - the Siemens TGN4 which started this idea off is based on +/-6v signalling, which I finally realise is actually v.28, not v.10.
The I-Telex system for Double Current works using the VF 0, 30 and 60 lines - with the 30 becoming the common connection, and the 60 being +ve, the 0 becoming -ve
Because the power supply works by voltage multiplication the off load voltages are in fact somewhat higher, more like +/-45V
I opted to try to drop these voltages down to suit the machine - this required some modification to the original board, to lift the original supply pins from the backplane connector out of the board, and to install Zener diodes and current limiting resistors to suit - my initial parts holding only had 5.1v diodes in stock, so I tried these, with 1.5k limiting resistors
Bench tests looked promising - with the card out of the I-Telex system and running on bench power supplies, it is possible to see the emitters of the switching transistors operate with an oscilloscope. Helpfully, the Atmel sems to run in some kind of loop when not connected to the I2C bus, so there are cycles of changed polarity.
While the card was on the bench, I also needed to modify the Keyboard connections - with only 6V at a few milliamps to play with, I removed all the 3K3 resistors that drive the sender optocoupler and substituted with just one 430R component - this should then mean about 10ma is available to drive the opto.
Card suitably "Bodged Up", time for a test.
Powered up in I-Telex - No Smoke..... Encouraging....
Connected a multimeter to check if there would beany polarity changes on the receiver output - Yes, switching is happening, good.
Connected the TGN4 (Siemens T1000).....
Yellow Lamp permanently on.... Okay, ignore that for now.... pop machine to online, and type a few keys - Green Light flashes - Cool, so I-Telex and machine are half talking.
Problem is, Yelo Lamp on ,means I-Telex thinks we are already in use.....
Sneak up on I-Telex - Power up with machine disconnected, press button on card,then plug machine in... Aha, Configuration page prints!
Configure for Keyboard dialling - Okay, this works.....
Manage an outgoing call to Henning, and that's where I have got to.
Things to do:
Understand why I-Telex thinks the machine is online, even when it isn't
Massively reduce dissipation - 1ma of current is enough - I had guessed 10ma
More to do - suggestions welcomed.
Cheers
Sean