PDP-8/L current loop -> RS232 help

No wonder you can't send any data, then. That would be interpretted as a constant 'mark'. A wiring error is a hardware problem ;-). More seriously, what I meant was that if you can't get the RS232 port to turn off the loop current, then there is no point invesitgatign the software on either machine.

OK, I am hopefully not insulting your intellegence too much here. I will try to start with the basics. Suppose we have 2 devices and we need to send an electrical signal from device A to device B. The obvious wa to do it is for device A to output a suitable voltage and device B to have some kind of voltage detector. That's what a voltage-level interface, like RS232 is. The other way is for deviec B to sense the current flowing through its input cicuit nad deviec A to somehow control that current/ That is a 'current loop' interface. As an aside, there is nothing that implies a current loop has to be a digital intterface. Just as a voltage-level interface can be analogue (a sensor can output a voltage depending onbthe quantity it is snesing), a sensor can also control the current in a loop. In fact many industrial sensors, at least over here, have a '4-20mA' output, they are designed for use in a current loop with a suitable analogue current detector. It is perhaps interesting to realise that if the sending device is perfect (that is, the voltage output in a votlage-level sender is independant of load, the current in the current loop is independany of the loop resistnace), then voltage level interface is unaffected by leakage between the wires, but the received voltage (assuming a non-infinite input resistae) does depend on the wire resistace whereas the current loop current is unaffected by the wire resistance but the current through the detector will be reduced by any leackage in parallel with it. In many cases leakage is a lesser problem than wire resistance, hence the use of current loops OK... Let's get back to digital current loops of the type you're tryign toi get to work. The current loop circuit is quite simple. It is a loop containing 3 items : A 'switch' to control thr current; a 'sensor' to detect the current; and a power supply. For practicla reasons it is sensible to put the power supply in the same case as one of the other devices. An current loop device (wheter sender or detector) with the power supply built-in is called 'active'. One without is called 'passive'. You link an active device to a passive device. That way you have the 3 things you need in the loop. 2 psssive devices linked together can't work, there is no power supply to send current round the loop. 2 active devices linked togethter is a bad idea, there are then 2 power supplies in series. The Teletype Model 33 is passive on both transmit and receive AFAIK, It is certainly passive on transmit. The transmitter is just a complex circuit of switch contacts. So anything designed to connect to that transmitter has to be an active interface (and yes, every DEC terminal interface I've looked at has been active). The fact you are getting loop current means you have a PSU in the loop too. That is, there is an active device in the loop. Since the RS232 interfce you are using, wired as you say you have wired it, is passive, the PDP8 must be active. I think, though, you don't need ot worry too much about 'current loop' if that's what is scaring you. You hve the scheamtic of the RS232 adapter -- it's in the document you pointed us at. I am sure you have schematics of the PDP8 side too. In which case you can analyse it like any other circuit. It's just transistors and simple ICs after all. Now for a dirty trick I learnt years ago. I notice there's an opto-isolator in the RS232 adapater between the RS232 input buffer and the current swich transistors. I don't know if the idle state of this opto-isolator is 'on' or 'off'. So for the dirty tricks : 1) You can see if the isolator should be on by measuring the voltage drop across the LED series resistor and seeing if the LED is carrying any current. See if that changes when you send a break. 2) The really dirty trick. If you want to force an opto-isolator off, short out the LED conenctions. If you want to force it on, short C-E on the output-side transistor. For opto-isolators in 6 pin packages, those are pins 1-2 and 4-5 respectively. See if doing either of those will reduce the loop current to near 0. The idea is to find out what section works and what doesn't. There aren't that many components involved. -tony