[GreenKeys] 20ma current loop for my Dad's KSR 33

[Ken Wiebe]

Jeff -

If it turns out you need to convert to RS-232 for your particular computer, I believe in order to do what you proposed with a TWX 33 you will need to do what GK'er Keith Lueck described to me in June:

>From Keith:

A "computer" 33 has electronics in it that translate a 20 mA loop current into signals that the selector magnet driver (SMD) can use - the magnet in a 33 actually requires a half amp. (In older teletypes like the model 15, the loop current drove the selector magnet directly.) In the case of a TWX machine, the 101C modem sends those signals to the SMD. A simple circuit can interrupt those signals and you can inject your own data. In a TWX machine the 101C modem occupies most of the base. In a "computer" 33, the base is empty. To access the modem you have to remove the rear panel of the base. 

Here's what I did:

Attached is a schematic of the 101C, and I've noted the signals and signal levels coming from the modem to the selector magnet driver and from the keyboard to the modem. The top set of connections is the data going to the modem from the keyboard. The bottom set are the signals going from the modem to the SMD. You can access these on the giant terminal strip on the far right side of the modem, inside its cabinet (you have to swing out one of the hinged modem sections to be able to get to the terminal strip) - "D66"refers to terminal 66 on that strip - they're labeled every 10 contacts or so. The wire coming from the keyboard distributor (or tape reader) is red with a black stripe (R-BL) and lands on terminal number 69 ("D69"), the wire leaving that terminal to the modem is white with an orange stripe (W-OR). All you have to do is interrupt those connections and wire in your level shifting circuit. The wire coming from the modem to the terminal block is blue with a white stripe (BL-W), and the wire going to the SMD is black with a green stripe (BLK-GR), 

Also attached is my level shifter, using a MAX232 chip and some solid state relays. It was designed to work with either my 33 or my (really old) 15. There are two different inputs depending on 60 mA hv loop from the 15, or +/-20V from the 33 keyboard. You could eliminate the loop input in your case. Just connect a wire from terminal 66 to the +/-20V input on the level shifter. I picked up ground from the frame of the 101C. The output goes in series with from the modem to the SMD - so you actually break the connection at terminal 69 and wire the "loop out" connection in series with the interrupted connection. The output, as noted on the schematic, is polarity sensitive - the SSR's work best (lowest output resistance) when wired for DC and not AC. So, "output +" connects to the SMD side of the circuit, and "output -" goes to the 101C side of the circuit. As I said in my original post, this could be simplified somewhat for use with only the 33, but I haven't gotten around to it yet... 

Addendum from Keith:

Ken - 

I made a couple typos [above]. The SMD circuit is at D66 - that's the one you have to break and put the SSR in series with. D69 is keyboard / tape reader data, and that's the one that you add a wire from there to the "+/-20V -IN" input on my circuit. Ground (modem frame) goes to "+/-20V +IN". 

Also, the wire colors do not change at the terminal block, as I'd implied - they change at a pair of 50 pin Cinch connectors (labeled "P" and "T" on the schematic) - so, the color I noted on the left (modem) side of the schematic is the color you'll see at the terminal block. On the other (right) side of the connector, which goes up to the typing unit, the colors change there, and if you're trying to trace the wiring inside the top side of the machine, those are the colors you look for there. 

On Mon, Oct 7, 2019, at 15:47, Jeff Albrecht wrote:
> This was my Dad's. Wayne says it is a TWX machine. Dad had it hooked up to an IMSAI ~ forty years ago, although all the details of his connections that apparently were made in one of the two connectors that look like they would be hooked up to a phone system have been lost. I'm pretty sure it was via a IMSAI SIO. I personally played Star Trek on it. 
> 
>  I'd like to connect Dad's KSR like I did with this ASR 33 I bought several years ago and hooked it up as shown on my wiki page; 
> https://wiki.theretrowagon.com/wiki/20ma_current_loop_to_RS-232_conversion#Image_details Theoretically should that wiring configuration work on Dad's KSR? The numbered screw strip in the second image?
> 
>  So I went to plug it into A/C to see how it did in Local mode. 2 of the 3 fuses (see last image) are labeled. The third that is missing it's fuse is not labeled. Any idea what fuse value I should place in there?
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