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. =/
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
