SWTPC 6800

Mike Stein mhs.stein at gmail.com
Fri Aug 12 20:44:39 CDT 2016

I'm still looking for my AC-30; as soon as I find it it's yours. See ya off-list...


----- Original Message ----- 
From: "Brad H" <vintagecomputer at bettercomputing.net>
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
Sent: Friday, August 12, 2016 9:17 PM
Subject: Re: SWTPC 6800

> Interesting. I thought the CT-1024 was sort of the intended companion for the 6800 (It came out first, I think). I wonder what they expected people to do if they had just those two devices?
> I'll probably try cable swap and see how onerous that is. I'm hoping to one day acquire an AC-30.. of course then I'd need to find tape files...
> Do you know of any good repository for the kind of loader files you can load via serial? I've found a few here and there but not all of them. 
> Sent from my Samsung device
> -------- Original message --------
> From: Chris Elmquist <chrise at pobox.com> 
> Date: 2016-08-12  4:01 PM  (GMT-08:00) 
> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org> 
> Subject: Re: SWTPC 6800 
> On Friday (08/12/2016 at 07:33AM -0700), Brad H wrote:
>> I've a question. I've now got my CT1024 working properly with my 6800.. is there an easy way to load txt loader files into it while it is still connected to the CT? Or do I have to load something in via PC first and then swap cables?
> The "usual" method in the day was that the paper tape reader on the
> M33 teletype connected to the 6800 as the console was used to load your
> s-records in through MIKBUG. When you started the tape reader, it was
> just like you were typing it on the TTY's keyboad.
> Later, a cassette interface such as SWTPC AC-30 or the PERCOM CIS-30 was
> used and it sat between the terminal's RS232 interface and the SWTPC's
> console interface. When you were loading a tape, the terminal got
> disconnected (electrically) and the data coming off the tape was sent
> to the console input of the 6800.
> So, in simple terms, the cassette interface was in series with the
> terminal and could preempt the terminal when loading from tape. To save
> to tape, the output from the 6800 would essentially go to both the tape
> and the terminal at the same time.
> The modern equivalent is probably an RS232 A/B switch that either
> connects your CT-1024 or a PC to the 6800's console. When you want to
> "load a tape" you flip the switch so that the PC connects to the 6800
> and sends the s-records in. After the load is complete, you flip the
> switch back and the CT-1024 becomes the console.
> You could also diode-OR the transmit data from the CT-1024 and a PC to
> the 6800's receive data input and wire the transmit data from the 6800's
> output to both the CT-1024 and the PC but this might be sketchy depending
> on the PC's RS232 interface characteristics. But I have done this
> successfully with other RS232 interfaces where I wanted two devices to
> be able to send to one receiver without having to physically disconnect
> or flip a switch.
> Chris
>> -------- Original message --------
>> From: Pontus Pihlgren <pontus at Update.UU.SE> 
>> Date: 2016-08-11 11:27 PM (GMT-08:00) 
>> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org> 
>> Subject: Re: SWTPC 6800 
>> Very interresting read, thank you Ethan.
>> /P
>> On Tue, Aug 09, 2016 at 10:55:54AM -0400, Ethan Dicks wrote:
>> > On Fri, Aug 5, 2016 at 2:58 PM, Chris Elmquist <chrise at pobox.com> wrote:
>> > > On Friday (08/05/2016 at 06:50PM +0000), tony duell wrote:
>> > >>
>> > >> Am I the only person who rarely, if ever, has RS232 problems?
>> > >
>> > > No. ;-)
>> > 
>> > No, but I used to manufacture sync serial hardware and have deep
>> > knowledge of how async serial in general, and RS-232/EIA works in
>> > particular, and still have all the test gear from 30 years ago. I get
>> > why people find serial comms frustrating and do not take my
>> > experiences as "typical".
>> > 
>> > I pretty much don't hook up anything new that isn't on a "traffic
>> > light". I have several - DE9-DE9 for modern stuff, and multiple
>> > DB25-DB25 for old and new stuff. *Mostly*, if you plug everything in
>> > and you get at least TxD and RxD to light up, you at least have
>> > figured out where the primary gozintas and gozoutas go and can stop
>> > adding null-modem adapters. Past that, you have to know if either end
>> > requires hardware handshaking and either plumb the right signals
>> > (vintage setup documentation is invaluable for this) or bridge the
>> > appropriate lines (documentation again) so that one or both sides
>> > _thinks_ there's hardware handshaking. If you defeat it, you might
>> > run into overrun conditions, but at least you should be able to
>> > establish basic comms and pass a few characters. To that end, you do
>> > have to match speeds on both sides, and the usual best place to start
>> > is 8-N-1 for data bits, parity, and stop bits. I've run into multiple
>> > situations where 7-E-1 or 7-N-1 is the right answer. With enough
>> > experience, the "baud barf" from mismatched speeds takes on an often
>> > recognizable pattern that can be used to quickly figure out what the
>> > speed ought to be, but lacking instrumentation like a serial analyzer
>> > or an oscilloscope, one can try "all the speeds" until cleartext comes
>> > through. I also try the speeds in "most popular order", 9600, 1200,
>> > 300, 2400, 4800, 19200, 600... in the hopes of saving time. Every
>> > once in a while, you run into some oddball stuff, like 9600/150, etc.,
>> > split speeds from the days of timesharing setups where the CPU wanted
>> > to get data to the users as fast as possible but wanted to minimize
>> > input interrupts and more closely match the input flow to (slow) human
>> > typing speeds. This wasn't common with microcomputers, but I've seen
>> > it with PDP-11 and PDP-8 setups and isn't something to look for first,
>> > but it does exist and highlights how strange things can get if all
>> > you've ever done is plug a high speed modem into a PC for dial-up
>> > internet.
>> > 
>> > One important tip about USB serial dongles (and some laptops DE9
>> > serial ports) - I've had problems with some of them and 1970s gear
>> > because the EIA/RS-232C (1969) level specification is +5V to +15V for
>> > space (0) and -15V to -5V for mark (1) (with +/-3V min sensitivity)
>> > and a lot of old gear is expecting +/-12V and not happy with
>> > lower-voltage lines and thin wires that don't help signal losses. One
>> > case in particular was a 1977-era Bridgeport Series II CNC mill with a
>> > LSI-11/03 CPU and a lot of custom Bridgeport boards. Everyone else
>> > who tried to talk to the Bridgeport before me had zero success. I
>> > checked all the things on the list and finally pulled out the laptop
>> > and set up a Dell desktop which worked the first time. When
>> > connecting to pre-1982 gear, I'd definitely try it from a desktop if a
>> > laptop is just not working. Checking the lines with an oscilloscope
>> > could also help verify this what's giving the grief (I did not have
>> > one handy when we were trying to get that CNC mill working).
>> > 
>> > In terms of serial analyzers, there are several types out there, and
>> > the ones that I've had the most time on are the HP 4951/4952 series.
>> > There are different models with different features, but if you are
>> > going to shop for one, ensure it comes with the "keyboard lid" because
>> > that's where the line drivers and serial connectors are. The large
>> > connector on the back goes to a "pod" that happens to snap on the
>> > front of the unit when the keyboard is flipped up. It's much easier
>> > to find the base units than loose pods, IME. Check which pod. I've
>> > seen many with DB25s, which is probably what you want, but I've also
>> > seen DC-37 connectors, and non-EIA (RS-232) level shifters. The good
>> > news is that among these different models, the pods should be
>> > interchangable, so if you end up picking up 2 units (not unusual) with
>> > different base capabilities (some have DC-150 cassette tape, some have
>> > 3.5" floppy, plus some minor differences) and only one has a DB25 EIA
>> > pod, you can at least migrate it between the units. I find the serial
>> > analyzers invaluable for snooping on the details of what's happening
>> > on the wire, but any analyzers I have seen have a handy "autoconfig"
>> > button to sniff traffic and configure the line for monitoring, so it's
>> > often a quick click to get all the parameters if you don't already
>> > know them. Where they really shine is looking for troubles at the
>> > application layer, debugging Kermit or XMODEM traffic that isn't
>> > working for any obvious reason. The advanced stuff you can do is to
>> > write programs for some analyzers to simulate a host or a client for
>> > software debugging or to reproduce a problem for deeper analysis -
>> > this is far beyond the usual "why can't I get this terminal working
>> > with this vintage machine" but when you need it, you need it.
>> > 
>> > In summary, I start by scoping the line with an LED traffic light
>> > (swapping lines or making custom cables where necessary), then move on
>> > the speed and parity settings (and changing the easier-to-change end),
>> > then look deeper when the easy stuff doesn't work. Easy problems take
>> > minutes or less. Hard problems can take a long time to resolve.
>> > 
>> > -ethan
> -- 
> Chris Elmquist

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