Was: PDP-8/A transformer hum Is: VXT-2000 stuff.

Joseph Zatarski joezatarski at gmail.com
Wed Jun 17 22:51:38 CDT 2020

> On 6/9/2020 11:51 AM, Glen Slick via cctalk wrote:
>> On Tue, Jun 9, 2020 at 9:04 AM Bill Gunshannon via cctalk
>> <cctalk at classiccmp.org> wrote:
>> For the VXT-2000 with an H7109-B the rated voltage and current values
>> are printed right on the power supply label:
>> +5.1V, 7.81A
>> +12.1V, 0.62A
>> -12.1V, 0.46A
>> -9V, 0.2A
>> Fixed width character pinout diagram:
>>        +=================+
>>    -9V | Yellow | Orange | +12.1V
>>        +--------+--------+
>>    ??? | White  | Black  | Gnd
>>        +--------+--------+
>> +5.1V | Red    | Blue   | -12.1V
>>        +--------+--------+
>> +5.1V | Red    | Black  | Gnd
>>        +--------+--------+
>>    Gnd | Black  | Black  | Gnd
>>        +=================+
>> The mystery is the White wire. The power supply label only lists 4
>> output voltages. The White wire appears to be routed to the Ethernet
>> daughter board. The measured voltage appears that it might be floating
>> slightly negative, somewhere around -1.5V when the Ethernet daughter
>> board is installed and around -5V when it is removed. Maybe it is a
>> high impedance earth ground connection? It appears to be connected to
>> the shield of the Ethernet BNC, which measures around 1M-Ohm to the
>> chassis ground when the power supply is disconnected from the main
>> board, and around 0.75M-Ohm when the power supply is connected.
> As long as we are talking VXT-2000, I have this on file, figure it would 
> be useful for future googling of the list if
> this were here as well.? May be a repeat of info from an earlier post, 
> or maybe elsewhere.
> I didn't save the source of the info, but looks like someone named Matt 
> Millman did it.
> The setup is as follows: HP Envy laptop running MOP boot daemon, 
> connected to Lantronix LTX-C twisted pair
> ethernet to AUI converter (not the same as a twisted pair MAU). This is 
> then connected to a cabletron coax
> MAU with a 10BASE5 vampire tap. The coax runs over to another MAU, which 
> is connected via an AUI cable to
> my VXT2000. I didn't think of it, but it would have been nice to have a 
> real DEC MAU on the VXT2000, I've
> got several DEC h4000's I could have used.
> https://www.youtube.com/watch?v=A5T2GlAN2N4
> http://tech.mattmillman.com/projects/10base5/

The first link is mine actually. (Hi, I'm Joe from Joe's Vax Repair and 
More). I have a VXT-2000 and have had it setup and booting on a few 
occasions. I also happened into some 10BASE5 equipment a few years back 
and the VXT2000 was one of the only AUI devices I had at the time. I'm 
not Matt Millman, but I did reference that site to find the existence of 
the Lantronix LTX-C and similar devices.

Now that I have more AUI gear, I plan to put different gear on the 
3-node AUI segment (cisco 2511 router, media converter to 10BASE2, 
VAXStation 3200) and move the other 10Mb equipment to a 10BASE2 segment. 
The whole mess will be joined to the main network via T1 on a Kentrox 
CSU/DSU on the 2511.

You can boot the VXT2000 using MOP or BOOTP and TFTP actually. MOP was 
the first thing I managed to get working, but BOOTP and TFTP worked 
equally well once I had it set up correctly. Unfortunately my bootptab 
file is on a roommate's MicroVAX II...

Anyway, see the links section here for the software: 

There's also theoretically OpenBSD and NetBSD support for the VXT2000, 
but in my experience the netboot stuff is pretty broken on VAX for 
whatever reason. There's at least one bug for the SGEC ethernet chip 
driver, and I could only get older versions of the network boot mop file 
to load. Of course, the older versions wouldn't support the newer kernel 
format, which apparently changed at some point. Or maybe I'm just doing 
something wrong.

If the machine in question gets repaired, feel free to contact me with 
any questions. If you're familiar with netbooting VAX systems, it's not 
too hard to figure out though.

I'll add that the note about the 9V supply being isolated sounds 
reasonable. It could be used as the supply for the built-in 10BASE2 MAU 
on some units. I can't confirm as my unit has 10BASET instead which has 
no requirement for an isolated supply.

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