Topics

HP 3081a ASCII terminals help

Richard Merifield
 

Hi All,
 
I thought I would try posting  sonething here, I have had no replies elsewhere.

I couldn't bear seeing some old HP gear getting thrown out so for $20 I purchased some old HP Industrial ASCII Terminals and bar code wands, model number HP-3081a. Some details here:
 
http://www.hpmuseum.net/display_item.php?hw=821
 
I was going to try and get the terminal interfacing with a PC via serial terminal software just for fun.
 
Anyway, as a way of background, the manual for the devices indicates the following info:
 
- The 3081A employs a 20mA Single current loop
- The 3081A terminal operates using a positive polarity 20 milliamp current loop
  Logic 0 (mark) = current (20 mA).
  Logic 1 (space) = no current (< 3 mA).
- The 308lA receives its power supply along the data cable from the interfacing unit
- The current loop circuit comprises a transmitter, receiver and current supply. The transmitter sends data by switching on and off the flow of current in the loop, this being detected by the receiver (see Figure A-1). The 3081A terminal's transmitter is connected to the user interface's receiver, and vice-versa.
Thereby, two independent current loops are formed allowing full duplex operations. In addition, the 3081A terminal acts as a passive receiver and passive transmitter, i.e. it "sinks" the current coming from the user interface unit
 
I am a little confused when it mentions "single current loop" and then ""independent current loops".
   
I have been doing some reading and thought the schematic for an old Teletype at https://oldfellowstoys.net/current-loop-to-rs-232-converter/ would work so I put it together. Some insight into what I should be seeing from this schematic would be a great help. If I measure between TX+ and TX-, and RX+ and RX- I see 20ma of current and about 11.5V on both. The HP3081a terminal runs on 12V to 36V DC.
 
When I connect this as I think is required to the 3081a I get nothing. The terminal wont power on, if I remove the 470ohm resistor from TX+ it powers on but this must surely affect the current. I think it has something to do with the "passive" nature of the HP3081a (i.e. loop powered) and my wiring in the circuit.
 
On the off chance that someone is interested and could provide some advice I have attached a few images from the 3081a manual. I am an absolute beginner but can make something with the right instructions.
 
Cheers
 
Richard
 

Martin Hepperle
 

Richard,

three years ago I obtained two of these, brand new, but with the simpler numeric keyboard (no alpha keys). No barcode wand, but a magnetic card reader.
I liked the 1970s brown/yellow/orange color scheme and the VFD display.

The current loop interface uses a 3-wire setup, so not all TTL-current loop converters may work because the input- output loops share a common line.
HP sold an interface box for these units which i do not have.

For an electronics engineer it is probably easy to interface these to a RS232C interface, but I needed more help and found some interface schematics in an old BYTE magazine.
This simple interface consists of a transistor and an optocoupler plus a -12V converter.
I was able to send characters and escape sequence to the unit so that they show up on the display, but have not implemented the receiver part of the interface.
With the simple numeric keyboard it is relatively useless - with the alpha keyboard you could indeed use it to log into a system and even run a line editor session.

I can dig out the BYTE article in the evening at home if that helps

Martin

Paul Birkel
 

Something like https://www.ebay.com/itm/Black-Box-CL410-Current-Loop-Interface-Module-powered-EIA-RS-232-20mA-101-4Q-Nos/111886999189 should do the job, but the BIN price is, in my opinion, much too high.  Even with free shipping.

 

From: VintHPcom@groups.io [mailto:VintHPcom@groups.io] On Behalf Of Martin Hepperle
Sent: Tuesday, October 15, 2019 5:38 AM
To: VintHPcom@groups.io
Subject: Re: [VintHPcom] HP 3081a ASCII terminals help

 

Richard,

three years ago I obtained two of these, brand new, but with the simpler numeric keyboard (no alpha keys). No barcode wand, but a magnetic card reader.
I liked the 1970s brown/yellow/orange color scheme and the VFD display.

The current loop interface uses a 3-wire setup, so not all TTL-current loop converters may work because the input- output loops share a common line.
HP sold an interface box for these units which i do not have.

For an electronics engineer it is probably easy to interface these to a RS232C interface, but I needed more help and found some interface schematics in an old BYTE magazine.
This simple interface consists of a transistor and an optocoupler plus a -12V converter.
I was able to send characters and escape sequence to the unit so that they show up on the display, but have not implemented the receiver part of the interface.
With the simple numeric keyboard it is relatively useless - with the alpha keyboard you could indeed use it to log into a system and even run a line editor session.

I can dig out the BYTE article in the evening at home if that helps

Martin

Richard Merifield
 

Thanks Martin. I have one numerical and one alphanumerical version. If you can find the article that would be great. I am too far into this to stop now :). I was going to see if I could use it to send code to my HP-71b via bar code, for no good reason.

I also like tHe retro colours. They are a work of art inside with perfectly laid out rows of resistors etc

thanks again

Martin Hepperle
 

Richard,

attached is the one page BYTE article by famous Steve Giarcia and another general article about current loop schemes.
I built the simple BYTE interface, which can be jumpered for various purposes. Today there are integrated circuits available which would simplify the interface, but it is not really complicated and it is always nice to meet a transistor or two.
The upper part worked for me (sending), but I did not get the receiving part right. Should be simple, but I put it aside due to other incoming projects.
Martin

Richard Merifield
 

Thanks Martin

I will see how I go, but this may be outside my capabilities.

I did end up getting the terminal to show something on the PC serial port but it just wasn't what I typed, i.e. A "B" on the 3081A might give a "Z" or something else like a "/" or ".".

Regards,  

Martin Hepperle
 

Richard,

I looked at the link with the two HCPL 4x00 current loop ICs.
This should work but for sending and recieving you would have to modify it so that a common ground return line is used.
This would require to rewire e.g. the lower transmitter (HCPL4100) so that the +12V---[470K]-- line goes to pin 3 and pin 4 would be TX+ while TX- would be connected to ground.
Then both loops (TX- and RX-) could share the common ground line.

Concerning the garbled characters: are you sure that baud rate, number of bits and stop bits and parity settings match?

Good luck,
Martin

Richard Merifield
 

Thanks Martin

No luck so far though. It is not clear to me how to connect the circuit (my original one) to the 4 wires from the HP3081a terminal.
It is easy to work out the following

Terminal 1 shield on 3081 to pin 9 rs232

Terminal 2 +V on 3081 to pin 8 rs 232

Terminal 3 RX on 3081 to pin 1 rs232

Terminal 4 TX on 3081 to pin 7 rs232

Terminal 0V GRND on 3081 to pin 4 rs232


The terminal needs about 12V/0.23A to turn on but the circuit only provides 20ma ?

The baud is set correctly

Richard