Help setting up old HP ASCII Terminal on 4-20ma current loop - Page 1
Forum section: electronics / beginners

author: RichardM
author_rank: Frequent Contributor
timestamp: October 13, 2019, 01:49:26 am
content: Hi All,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=821I 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 loopLogic 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 unitI 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 athttps://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.CheersRichard

author: stevenp
author_rank: Newbie
timestamp: October 17, 2021, 12:32:29 pm
content: Hi Richard,I wonder how far you've progressed on getting your terminals working with RS232 ?I have three of these HP3081A terminals with bar code wands. Never really knew what to do with them.....I always found current loop interfaces a royal pain in the @ss.Based on your posting, I thought I'd pull out my terminals and have a play around...If I supply +12dc into the DB9 connector (pin8=+12vdc, pin9=GND), I can get it to power up. It uses just over 200mA. Not sure how that's supposed to work on a 20mA loop..... This also explains why you could get it to power up when shorting the 470ohm resistor.Next, I found that the CPU inside is an Intel 8031.The 8031 has on-board UART and the connections are pin10 for RxD and pin11 for TxD.It should be a simple matter of cutting the tracks around pins 10 & 11 to isolate them, and then use a MAX232 or similar to convert the TTL from the CPU to RS232.I haven't tried this, as I don't have an immediate need to use the terminals, but I would say there's a fair chance it will work out.Sorry, upon closer examination, isolating pins 10 & 11 may not be as easy as first thought. The PCB traces are embedded in an internal layer. You'll have to use a continuity tester to buzz-out the traces.... I'd start with the 6N136 & 6N139's....A brute-force way would be to cut the pins on the 8031 really close to the PCB and bend them out to isolate them from the board and solder wires directly to them. Slight less brute-force would be to un-solder the 8031 and put in an IC socket with pins 10 & 11 bent out, etc....I'd be interested to hear how you go.Cheers,-stevenp

author: CaptDon
author_rank: Super Contributor
timestamp: October 17, 2021, 01:50:24 pm
content: It is not a 4 to 20ma. loop. It is a make/break 20ma./0ma. loop and was common withthe older teletype demodulators and ascii machines at 110 baud. The Intel SDK-85 asan example could be configured as 20ma. loop or RS-232 levels. Old Teletype Corp KSR-33and ASR-33 units were 20ma. loop and used mechanical commutators to generate theserial loop data. The 4-20ma. loop in your post is a purely analog signal path representing0 to 100% of whatever the 'detector' was measuring. Typically pressure transducers inoil fields used the 4-20ma. loops as they were less likely to cause a source of ignition duringa fault. There just wasn't enough energy in the arc to initiate a flame front.