Re; What to look for in a Model 33 Teletype - a newb's buyer's guide?

tony duell ard at
Fri Nov 21 14:15:42 CST 2014

> >
> > If you have a working UNIBUS PDP-11, there are still 20mA serial interfaces
> > avalable for it. I have a DL11-C (M7800-YA) you can have for what I paid
> > for
> > it (a song, just about), and someone on eBay has a DZ11-C (M7814) for sale
> > for cheap; someone else there has the breakout panel for it.
> I have virtually no idea what any of that means. =/


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

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). 


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