[GreenKeys] A reliable current-loop-to-RS232 converter

Justin Scott tty at impakt.net
Sat Aug 15 18:10:03 EDT 2015 

Right, my point being - the way his interface was saying to connect the teletype seemed wrong for how the teletype should have been configured.  Guess
I missed the part where he got it from you, Wayne- Certainly I would have realized it was configured correctly for how he was going to use it. :)

cheers,
j

teletypeparts at comcast.net(teletypeparts) wrote:
> Justin, 
>   
> Thanks for your input.  Gabriel's 33   is   configured for full duplex, 20MA, 4 wire connection.  That's the way I sent it to him unless I made a mistake, but don't think so as the cable on the UCC-6 was already in place and likely was used on a computer before.  I just checked the connections and labeled the line cord end.  20MA is easy to check, just look at the large green resistor in the UCC-6 and the left tap should have a wire on it.  60MA, the wire goes to the inside tap.  I also tested the machine with a signal generator at 20MA, full dux.   
>   
> I don't want to confuse the issue if possible.  The 33 should work to a RS-232 adapter.  The problem is there are so darn many and they don't have a good track record.  I have seen lots of 33 owners struggle with this issue.  It almost always came down to the RW-232 adapter. 
>   
> Best regards, 
>   
> Wayne 
> 
> ----- Original Message -----
> 
> From: "Justin Scott" <tty at impakt.net> 
> To: "Gabriel Egan" <mail at gabrielegan.com> 
> Cc: greenkeys at mailman.qth.net 
> Sent: Saturday, August 15, 2015 3:32:33 PM 
> Subject: Re: [GreenKeys] A reliable current-loop-to-RS232 converter 
> 
> Gabriel, 
> 
> I think what maybe has been missed so far in this thread is that any configuration change you make to the interface must also be reflected in the 
> configuration of the Teletype.  On the Model 33, at the terminal strip in the back of the UCC, there's a very specific way these wires should be used. 
> My Model 33 came to me configured for Half Duplex, 60mA.  That had to be changed to Full Duplex, 20mA.  This is all laid out in the manuals, and 
> there's even some great websites out there with knowledge about how to interface teletypes to computers. 
> 
> The thing about using 3 wires instead of 4 wires for the actual loops sounds quite a lot like half duplex configuration to me, which you should not be 
> using with your Altair. 
> 
> When I first got my 33 running, I needed a way to get tape data off the internet and onto tape... so I quickly whipped together this interface: 
> https://www.circuitlab.com/circuit/mnartk/screenshot/1024x768/ 
> 
> To make the 12v, I simply used a full-wave bridge rectifier built out of a handful of 1N4001's and a small electrolytic to smooth it all out. 
> 
> Since I didn't have any 1/4 watt resistors at around 600 ohms, I used a pair of 1/8 watt 1.2k resistors in each loop for the current limitation. 
> 
> I built the whole thing up on a solderless breadboard, and used it for a couple weeks until I finally broke down and bought a BlackBox CL050 
> interface for about $20 on ebay, which works perfectly when configured correctly (the manual is still on their website, along with schematic). 
> 
> On the circuit I've outlined above - one thing to keep in mind is that on the 232 RX side, it will not drop below 0v for obvious reasons (i.e. will not 
> go to -12v).  Most 232 interfaces are fine with this, and this circuit does work with TTL level 232 signals. 
> 
> Here's a photo of that circuit on breadboard: http://i.imgur.com/DIKYPez.jpg 
> 
> There's a lot of ways that CL can be configured.  Making sure what you have configured on the interface is mandatory to match what you have the 33 
> configured for. 
> 
> Hope this helps! :) 
> 
> cheers, 
> j 
> 
> 
> Gabriel Egan(mail) wrote: 
> > I'm trying and failing to get my new ASR-33 
> > communicate with my Altair 8800 clone via 
> > a current-loop-to-RS232 converter purchased 
> > from the usually reliable UK retailer RS 
> > Components. 
> > 
> > The converter has rather a lot options, and 
> > it's able to support Active as well as Passive 
> > current-loop operation. I've got it set to 
> > Active and it's got 5v coming in from a USB 
> > cable. 
> > 
> > On the Teletype end, I have four coloured wires 
> > coming from the Teletype: 
> > 
> > TTY Send: Red (3) and Black (4) wires (not 
> > otherwise differentiated) 
> > 
> > TTY Receive: White (7) (positive) and 
> > Blue (6) (negative) 
> > 
> > (These assignments including the numbers 3, 4, 6, and 
> > 7 come from a label on the data cable coming from the 
> > Teletype). 
> > 
> > On the converter I have a terminal block with 
> > five screws. If I select Full Duplex operation 
> > and select that both Tx (Transmit) and Rx (Receive) 
> > are Active then those five terminals have these 
> > assignments: 
> > 
> > TB1: Tx I+ Out / V+ 
> > TB2: No Connection 
> > TB3: Rx I+ Out / V+ 
> > TB4: No Connection 
> > TB5: Tx/Rx I- In / V- 
> > 
> > (This is exactly how the manual describes them; 
> > I assume that "I" means "current", as Andre-Marie 
> > Ampere himself decreed.) 
> > 
> > On the principle that what the converter considers 
> > to be the act of 'sending' is what the Teletype 
> > considers to be the act of 'receiving' I connected 
> > the White wire from the Teletype to TB1 and the Blue 
> > wire from the Teletype to TB5. I set the converter 
> > to DCE operation and booted the Altair: Success! The 
> > MITS BASIC welcome message prints on the Teletype 
> > and I'm asked to enter Memory Size, just as 
> > expected. 
> > 
> > So, just the other two wires to connect and all 
> > should be well. This is where I get stuck! 
> > 
> > I connected the Red wire from the Teletype to the 
> > TB3 and the White wire from the Teletype to 
> > TB5. No luck. I reversed them: White to TB5 
> > and Red to TB3. No luck. Then I tried all 
> > permutations of the remaining terminals 
> > (everything except TB1 and TB5 since these 
> > are clearly in use for data going the other 
> > way). No luck. 
> > 
> > Then I did all the above again with the converter 
> > set to Active on the Tx side and Passive on the 
> > Rx side. (No use setting the Tx side to passive 
> > as this just makes the Teletype 'run open'.) 
> > 
> > What else might I change on the converter? 60ma 
> > instead of 20ms? No luck (Teletype just 'runs 
> > open'). Half Duplex instead of Full Duplex 
> > (No good: seems to be for systems sharing the 
> > same pair of wires for both sending and receiving, 
> > and we know we're not doing that.) 
> > 
> > I think I've run out of options with this particular 
> > converter. Is a 5v USB supply not enough current? 
> > This seems unlikely since they sell this converter 
> > for current-loop applications. 
> > 
> > If anybody on the Greenkeys lists can see my fatal 
> > error(s) in the above, I'd be grateful to know. 
> > 
> > Alternatively, can anybody point me towards the 
> > seller of a current-loop-to-RS232 converter that 
> > is known to work with Teletypes and for which 
> > they can tell me exactly where my four coloured 
> > wires go? If anybody is willing to make me such 
> > a converter, I'd be happy to pay any reasonable 
> > price. 
> > 
> > Regards 
> > 
> > Gabriel Egan 
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