Tinkering inside the ASR-33 is the most complicate part of the project. Here some pictures ... your ASR-33 may look different.


The Paper Tape Reader is enabled via a relay board (and the tape sensor, LOCAL/LINE and START/STOP). This relay is driven by the READER ENABLE signal via two wires.
"Orange" is READER ENABLE+, "Blue" is READER ENABLE-. They where cut-off internally in my ASR ... no idea which plug is normally used.

ASR 33 reader enable


The  relay coil is rated for 12VDC, but its specified to be driven with 15V, so they have a resistor of 120 Ohm onboard.
Computer driver boards like DECs DL11-W drive it with 15V.
I found the relay to work with 12VDC via 120Ohm too ... which is good, as tty2pi does not generate 15VDC.


P2 for Serial Interface and Power

 Serial signals and Power can be tapped at the internal "P2" connector ... thanks to deramp again.

ASR 33 back full


ASR 33 connectors

"P2" is the right of these two 3D printed Molex cases (upper left). The Molex part number is 03-09-2151, mouser.com have them, 538-03-09-2151. I decided to print my own (bad priority managment!).
Pins are the same as Molex drive connectors use.

Here pinout of P2 :

|15 14 13|
|12 11 10|
| 9 8 7|
| 6 5 4|
| 3 2 1|

Btw: The Molex connector left of P2 is a dummy ... just used to have a mounting point for READER ENABLE wires.

Cable ASR to TTY2PI

With Serial Data, Power and READER ENABLE we need an 8-wire cable.

ASR 33 cable plug


ASR-33 tty2pi Function My wire color
P2.3 J1.1 48VAC (68VDC when rectified) red
P2.4 J1.2 GND = 48VAC COM blue
P2.5 J1.3 GND = Data from TTY - grey
P2.6 J1.4 Data from TTY + pink
P2.7 J1.5 GND = Data to TTY - brown
P2.8 J1.6 Data to TTY + white
RDR ENB+ orange J1.7 Relay drive current+ yellow
RDR ENB- blue J1.8 Relay drive COM green


ASR 33 cable detail