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HP LX200 null modem serial connection

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First post, by sonictruth

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Hi,

I have an HP LX200 that I'm trying to connect to a PC using a USB to serial adapter. The settings are 300 baud, 8N1, no flow control, using 3 wires: RX, TX, and GND (2, 3, 5).

On the LX200 side, I'm using the Data Comm terminal, and on the PC, I'm using Minicom. The problem is that if I connect LX200 RX to PC RX and LX200 TX to PC TX, I get data in both directions, but everything is scrambled like this, for example:

a=O, s=F, g=L, o=H.

This is consistent. I have tried another USB serial adapter as well.

If I do a loopback test by wiring TX to RX, both serial ports work well.

If I reverse the connection to LX200 RX to PC TX and LX200 TX to PC RX, I get no data in any direction. However, if I remove the ground, I get correct data from the LX200 to the PC but no data the other way.

I'm not sure what is happening.
Is the LX200 serial interface broken ?
Are the hardware flow control wires required for this kind of connection ?
What else could i try ?

Thank you.

Reply 2 of 7, by DaveDDS

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Sounds like a baud-rate or data format (bits/parity) mismatch.

If you have the ability to run 16-bit code (DosBox works well), I'd suggest
trying my SDT (Serial Debug Terminal) - it will let you control/see a lot more details
about a serial port.

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

Reply 3 of 7, by DaveDDS

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jmarsh wrote on 2025-04-01, 12:31:

USB Serial adaptors can have very "loose" timings at low/high baud rates. Try something more standard like 19200.

Another thing I've seen in a number of them - some don't properly support RS-232 levels, which
should be + and - (3-25)v. - I've run into a few cheap ones which don't do power conversion and
therefore have only +5 and 0 v.. 5v is a valid RS-232 signal, but 0v isn't! and can confuse some receivers!

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

Reply 4 of 7, by sonictruth

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I also tried a WiFi modem installed on a WEMOS LOLIN D1 mini clone. All the RX TX combinations.
I've seen that this should work, but perhaps all the pins need to be connected or i'm missing something
or the serial port inside the LX is malfunctioning in an unusual way.

https://www.reddit.com/r/interestingasfuck/co … tible_computer/

Reply 5 of 7, by Jo22

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A MAX232 chip can help doing voltage conversion, for TXD, RXD and GND it's good enough.

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 6 of 7, by sonictruth

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Thank you. The culprit seems to be the cheap USB adapters.
I managed to connect the LX200 to a late '90s Omnibook using a 3-wire null modem cable, and it worked perfectly. I will buy a MAX232.

Reply 7 of 7, by DaveDDS

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Just be aware that a MAX232 is an interface chip: Digital Logic <> RS-232
This does both a level conversion and an invert (+in = -out) it is intented to
connect between a UART chip and the RS-232 serial port.

if the USB "serial" adapter has a board mount "pins" on the serial side, these
are likely simple digital logic in and out - and it may well be compatible with a
MAX232

If the serial cable has a DB9 or DB25 connector on the serial side, it may not be directly
compatible with the MAX232 - probably the wrong polarity, and depending on what exactly
they have done with level shifting ... may not be compatible voltage wise.

If you have a multimeter, I can give you some simple tests do help determine exactly
what it's interface characteristic are..

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

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