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From: Clem Cole <>
To: Warner Losh <>
Cc: segaloco <>,,
	The Eunuchs Hysterical Society <>
Subject: [TUHS] Re: AT&T UNIX PC (7300) Teardown and Restoration
Date: Wed, 4 Jan 2023 15:48:02 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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Matt - send me an email offline if you need more help.   Warner speaks the
truth - although he left out some wizardry that has sadly been lost for

The first board he offers will create the interface that an IBM PC/AT
created which was originally not a standard either (eventually EIA back fit
it, in RS 232-E IIRC, but RS-232 C which is what the UNIX PC's interface
was defined does not use it].

From your email, it looks like AT&T out a standard Data Terminal Equipment
(DTE) pinning on the back of that system (which would make sense). That
means it should be a >>male<< 25 PIN (i.e. male) D style 'B-sized'
connector with pins 2 (TX), 4 (RTS), 20 (DTR) being driven; and pins 3
(RX), 5 (CTS), 6 (DSR), 8( DCD), 22 (RI) as inputs.  With Pin 7 being the
signal reference ground and Pin 1 being the frame ground (often N/C - but
do not short to 7 on your board)

The signals should swing from +3 volts to up to +30 when assert as a
spacing time, and  -3 to -30 volts marking time. However, anything over +/-
15 volts is unusual, but the interface is required to accept it.
 Frankly, +/- 9 volts is most likely what you will see with the 1488/1489
driver pairs which were the preferred chips used in those days.

Amazon does not seem to sell an equiv that spits out real RS-232C using the
D connector, but rather this PC/AT variant, so you will need to make a
cable -- the good news is they do sell:

My suggestion would be to use this on you Mac/WIndows or Linux box:
connect it to the cable I just mentioned -- at that point your
Mac/Windows/Linux Box will look like a Proper 'terminal' as the being
expected then if needed

a 25 pin  null modem between it that the 3B1 such as: or
whatever one you need M/M, M/F -- etc...

On Tue, Jan 3, 2023 at 7:58 PM Warner Losh <> wrote:

> is my go-to for when I need a simple one, though something like
> might work better for your setup if you just have more of the TTL gear
> than RS-232 gear (I have way too many old RS-232 cables because I never
> know when I'll need them again...)
> Warner
> On Tue, Jan 3, 2023 at 5:46 PM segaloco <> wrote:
>> Nope, unfortunately I only know as much as is needed to plug into the
>> GPIO RX/TX/GND working on single boards. I wouldn't be surprised if my
>> assumption I just run these pins into the D-25 in the right holes is
>> bunk...but that's how we learn sometimes. Once I move I should be able to
>> setup a proper work bench where I can tinker on this sort of thing more
>> intentionally rather than haphazardly. So perhaps a silly question for an
>> expert but a valid one for me. I'll chase that angle and see if it bears
>> fruit.
>> - Matt G.
>> ------- Original Message -------
>> On Tuesday, January 3rd, 2023 at 4:40 PM, Warner Losh <>
>> wrote:
>> On Tue, Jan 3, 2023, 5:30 PM segaloco via TUHS <> wrote:
>>> I'd love to get my hands on a 3B2 someday, this'll be cool if I can get
>>> it going but that'd be a much more robust machine.
>>> I'm starting to suspect if there isn't any sort of boot ROM that spits
>>> out commentary on the UART and that doesn't get flexed until UNIX is up, I
>>> may not be able to get very far. I referred to
>>> for the serial settings and it appears:
>>> 9600 baud, 1 stop bit, no parity, 8 data bits
>>> And the relevant pins
>>> Pin 1 - GND
>>> Pin 2 - RX
>>> Pin 3 - TX
>>> Pin 4 - RTS
>>> Pin 5 - CTS
>>> Pin 6 - DSR
>>> Pin 7 - GND
>>> Pin 8 - DCD
>>> Pin 20 - DTR
>>> So I've plugged my USB-TTY GND/RX/TX into the relevant pins and setup
>>> the necessary tty settings. The manual then suggests if running null modem
>>> mode to short pin 4 to 5 and then pins 6, 8, and 20 together, presumably
>>> omitting any need for modem signalling from the remote machine, doing basic
>>> serial RX/TX. Unfortunately even with all of this bypassing I get nothing
>>> out of the RS-232 port. What I don't know is if I could even expect
>>> something or if this is unlikely to bear fruit whether the hardware works
>>> or not. In any case, if I do get this thing running I'll have a writeup for
>>> folks afterwards. If not, then hopefully I can figure out something useful
>>> to do with this thing rather than junking it.
>> Silly question... did you level shift to RS 232 levels? You likely did...
>> but if not...
>> Warner
>> - Matt G.
>>> ------- Original Message -------
>>> On Tuesday, January 3rd, 2023 at 3:53 PM, <
>>>> wrote:
>>> Hello Matt,
>>> I’ve got one of these in my garage. I bought it about twenty years ago
>>> as a working system but when I got it home I noticed that the hard disk
>>> wasn’t connected but at some point I’d like to get it and my 3b2/300
>>> working.
>>> Regards, Rob.
>>> On 3 Jan 2023, at 23:27, segaloco via TUHS <> wrote:
>>> And here are some pictures of the guts.
>>> Various bits inside date this to late 1985. The good news is it at least
>>> turns on, but that's about as far as I've gotten with it. The display never
>>> turns on, nor do I hear any sounds indicating it tries to start the CRT.
>>> The fans kick on and there it stays until I turn it off. I plugged in a
>>> USB-TTY to pins 2, 3, and 7 (RX/TX/GND) and listened at 9600 baud 8 bit 1
>>> stop no parity and got nothing. Swapped the RX/TX, still nothing. Of
>>> course, that's all predicated on the assumption there's something there to
>>> even interact with. I have little faith that whatever UNIX install was on
>>> this is extant. Additionally, it didn't come with a keyboard, so if there
>>> was some futzing with key combos that would trigger some sort of UART over
>>> those lines, I can't do that. I wonder if there are some contacts inside I
>>> can just poll for activity with this serial connector, not sure how safe
>>> that is...
>>> Anywho, the CPU has a bit of corrosion on the surface, not sure how that
>>> bodes for the innards, but this is in kinda rough shape either way. I hope
>>> I can salvage it but if not, I'm going to at least do some study on the CRT
>>> particulars and see if I can extract and keep the monitor from it, been
>>> wanting a smaller CRT to have around for a while.
>>> - Matt G.
>>> ------- Original Message -------
>>> On Tuesday, January 3rd, 2023 at 12:20 PM, segaloco via TUHS <
>>>> wrote:
>>> Good day everyone, just starting a thread for yet another project I'll
>>> be tinkering on over time. Picked up a (presumably broken/untested) 7300
>>> off eBay to at the very least tear down and get some good pictures of and,
>>> with some luck, perhaps get working again.
>>> Here are some pictures of the exterior for starters. I'll update this
>>> thread when I've got pictures of the guts and also with any info I can
>>> glean regarding whether this might be salvageable. The rust on the back is
>>> pretty nasty but I've seen older/worse start up just fine.
>>> - Matt G.

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  parent reply	other threads:[~2023-01-04 20:49 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-03 20:20 [TUHS] " segaloco via TUHS
2023-01-03 23:27 ` [TUHS] " segaloco via TUHS
     [not found]   ` <>
2023-01-04  0:30     ` segaloco via TUHS
2023-01-04  0:40       ` Warner Losh
2023-01-04  0:46         ` segaloco via TUHS
2023-01-04  0:57           ` Warner Losh
2023-01-04 12:39             ` Ori Idan
2023-01-04 20:48             ` Clem Cole [this message]
2023-01-04 20:53               ` Clem Cole

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