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

Sat Aug 15 19:02:08 EDT 2015

To convert the TTL level RS-232 signals to a proper bi-polar signal, you
only need a MAX232 chip (or clone) and 4x 1uF @ 16V capacitors. The MAX232
directly converts back and forth between 5V TTL and the more correct +/-12V
 signaling.

Easy as pie, and the chips are like a buck or something. Let me know if you
need one, I have a couple spares.

On Sat, Aug 15, 2015 at 3:24 PM, <teletypeparts at comcast.net> 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
>
> ------------------------------
> *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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