Tuesday, July 10, 2018

ASR 33 input received perfectly, can't get PC to RS232 link to output anything


The model 33 is performing nearly perfectly already. The sticking that occurs where you a keypress isn't recognized has dropped to perhaps one in fifty presses and it recovers faster. I expect that with just an hour or so of use the stickiness will totally disappear.

I built the MAX 3232 board onto a bit of PCB breadboard, along with a header that will fit onto the Altairduino and a zener diode regulator to drop the 5V from that socket down to the 3.3V needed by the board. The SBC6120 runs on 5V, thus I build a quick socket for that system to plug in my MAX3232 based board.

Alas, the MAX board was so small and fragile, with tiny flat pads to solder on wires, that it broke before I was able to use it. I did have a different and slightly larger board in a different project box, which I rescued and hooked up.

Armed with my RS 3232 board which converts 3.3V RS 232 signals to true RS 232 (approx +/- 12V), I tied together a chain of modules to link my PC to the ASR 33.
  • On the PC, I ran the PUTTY terminal program through a USB to serial unit based on the Si2102 chip, modified to run at 110 baud when the PC selects 1200 baud rate. 
  • The si2102 provides RX, TX (and other RS232 signals such as DTR) which I wire to my MAX 3232 board. 
  • The other side of the MAX board produces RX and TX as true RS232 voltage levels. 
  • Those +/- 12V signals are connected to the Telebyte 65 converter, which turns the voltage levels into the presence or absence of a 20ma current on its output loop. 
  • The current loop is connected to the teletype where it drives the printer and paper tape punch
  • The Telebyte also provides a 20 ma current loop to the teletype for the transmission side, the keyboard, answer back drum and paper tape reader
  • This second 'send' current loop comes back as RS232 data, converted to TTL levels and sent back to the terminal program over the USB lnk
First, I tested out the Telebyte unit. I hooked up an ammeter and verified that it is supplying 20ma current on the two loops (receive and send) that will be hooked to the teletype. Next, I checked across the RX, TX and ground pins to see that pin 3 is output when in DTE mode and pin 2 is output for DCE mode.

I wanted to get the polarities correct for all three units - ensuring that the USB module delivers data from the PC outward on the RX pin, that this is the input to the MAX3232 and the RX pin output of the MAX is delivering that data into pin 2 of the Telebyte where it will modulate the receive current loop.

It took a few tries to get the correct configuration of the Telebyte 65, before I turn on the ASR 33 in Line mode, have it humming quietly in idle mode. It now sits in idle mode when switched on to Line, with the keyboard sending data without it appearing on the printer so I have achieved full duplex operation as intended.

Next up is to get all the RS232 wires hooked in their proper orientation. For example, the CP2102 USB to RS232 module will appear to receive data on the TX pin (since when I brushed it to ground the PUTTY screen showed invalid characters arriving. ON the other hand, the MAX3232 board has TX shown as inbound to the RS232 side, thus I have to cross RX and TX between these modules.

I then see that the RX pin, pin 2, of the RS232 side coming out of the Telebyte module is active (-13V for the MARK or idle input condition). Therefore , this pin is going to be the input into the RS232 voltage side of the MAX board, the RX pin.

This means that data from the teletype keyboard flows out of the current loop, through the Telebyte and out of pin 2(RX). That pin should be hooked to pin 2 (RX) of the MAX3232 side for RS232 voltages. It will deliver the data on the RX pin on the TTL/3.3V side. That data from the RX pin of the MAX is crossed over to enter through the TX pin of the CP2102 USB module and is transferred up to the PC.

After some wiring of the RS232 cable I have, to link the pins on the Telebyte to the pins on the MAX3232, I could run a test trying to send and receive characters between PUTTY on the PC and the keyboard/printer. My initial setup is a maze of jumpers, alligator clips and other temporary scaffolding but eventually it will be proper connectors.

I can see the keyboard input modulating the "TTL" side of the MAX3232, and the keystrokes are immediately rendered correctly on screen by PUTTY. However, nothing I type on the PC is received at the teletype. The problem is somewhere in the USB module, device drivers, Windows or PUTTY. I don't see the little LED on the USB module flash when I try to transmit data back to the teletype, so it isn't even trying.

I made a try at hooking up the Altairduino, which I had worked on earlier to remove the bluetooth module and use it as a native serial port. Unfortunately, it is not working properly now. I can't get it to reset properly or configure or speak over the USB link. Having reached the end of the day working outside in the heat, I packed away everything and will try again tomorrow. 

I am in contact with a supplier of teletype parts, RTTY Electronics, and will be able to buy every part that is missing on my system except for the spring holder for the paper tape punch. Marc will machine a replacement for me.

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