OK... Unibus is the bus used by older PDP11 computers (and VAXen). It is similar in concept to Qbus (as used on later PDP11s and some VAXen), but the major difference is that Unibus has separate pins for the Address and Data signals, Qbus multiplexes them onto the same pins. Needless to say you can't plug a Unibus card into a Qbus machine or vice versa. There was a official DEC interface (DW11-B) to use Qbus peripherals on a Unibus CPU, and I think third party interfaces to go the other way. A PDP11 is a DEC 16 bit minicomputer. But I guess you knew that. The DL11 is the common single serial port for Unibus PDP11s. I think all versions can do current loop, most can also do RS232 (with the right cable wiring). The ones that are current loop only can often be converted to having the RS232 capability by adding some common ICs '20mA serial interface' means much the same thing here as 'current loop'. The RS232 interface signals the 2 states of a signal line (0 and 1, mark and space, whatever you call them) by a change in voltage on the interface pin. A current loop interface uses a change of current (here either 20mA for 'mark' and 0mA for 'space') to do the same thing. The 'current loop' is a simple electrical circuit containing a current source, a transmitter (which switches the current on and off, and in the case of the ASR33 really is a (very complex) mechanical switch) and a receiver (which senses the current flowing in the loop). In many cases the current source is built into the same unit as either the transmitter or receiver, which is then termed 'active' (one without the current source is called 'passive' and the rule is that you connect an active device to a passive device A DZ11 is another Unibus board, this time an 8 channel serial port. It shares a small number of I/O addresses between all the ports and can be a bit of a pain to program at the bare metal level. IIRC the DZ11-C is a current loop one -- there are different boards for RS232 and current loop here and it is very difficult (read 'impossible') to convert one to the other. The 'breakout' panel, aka 'Cabkit' (Cabinet Kit) is a panel that takes the ribbon cable from the DZ11 board (here) and brings it out on more convenient connectors, one for each port (I think sets of 4 screw terminals, 2 for the input loop, 2 for the output loop for each port). -tony

I think we're assuming you are going to connect this ASR33 to something else. ASR33s have a 20mA (or 60mA, link selectable, I think) current loop interface. Not RS232 (OK, I mentioned the Data Dynamics machine that used ASR33 mechanisms, but strictly that's not an ASR33). The most common machine (I think) with a 20mA current loop interface is a Unibus PDP11. Hence all the comments, if you had one of those, you could easily link the ASR33 to it. That said, a quick google search for 'RS232 current loop' (no quotes) throws up a number of commerical units and built-it-yourself schematics to convert the current loop interface to RS232. Pun intended? -tony

I suspect that most, if not all, of those RS232-TTL converters are just a MAX232 + the necessary capacitors, or something very similar. Of course I've not tried it but I can see no reason why it wouldn't work. On the other hand, I can see no reason not to just buy the MAX232 chip, since you'll have to mount and solder up the optoisolators and associated components anyway. -tony

Possibly data format or baud rate translation. Or perhaps the TTL interface is not simply the asynchronous serial signals, but something like SPI. In any case you don't want it here. [..] Classic computers with current loop interfaces often opto-isolated them using 'any old optoisolator' As you said, it's only 110 baud, so speed is not an issue The current-loop -> RS232 side is quite easy, with 20mA available just about any optoisolator will work (you may even need to shunt some of the current round the LED, check the maximum If of the LED in the device you are using). The RS232 -> current loop side might be a little more work, in some cases the optoisolator's phototransistor won't pass 20mA. You may need to add an external transistor (essentially a darlington pair circuit). -tony

Howdy, On Fri, Nov 21, 2014 at 3:57 PM, Fred Cisin <cisin at xenosoft.com> wrote: Thanks for that info, Fred - but we're really off-track, here.. ;-) This all relates to (what now seems to be) my inevitable future acquisition of a Model 33 Teletype - and how to deal with issues such as making +it+ talk the RS-232 talk. Not only do I not actually have the 33 yet, but I don't even have a (functional) serial card for my Altair 8800. If I can figure out how to use my existing serial card, it may be a moot point, as it seems my card might actually support the 20mA standard. But if I end up with an RS-232 only board, then I want to be at least semi-prepared to work with it; armed with the knowledge & documentation, if not the hardware as well. (I find that if I don't rush-rush everything, and plan a bit in advance - acquiring what's required as opportunities present themselves, rather than at the last minute, when I +NEED+ to have it - that I tend to spend a whole lot less money, and waste a lot less.)