34401A can send over RS232 but not receive?  - Page 1
Forum section: electronics / repair

author: OsciX
author_rank: Contributor
timestamp: February 03, 2022, 06:18:10 pm
content: Hey guys, I have two 34401As here, and both seem to have the same issue with RS-232 communication. I can send commands (as evidenced by manually triggering, entering remote mode, and resetting) but not receive (IDN, READ?, all echo nothing). I've checked all the meter settings, and I'm using Putty to send. I feel like I've tried everything. Does anybody have any advice?

author: fzabkar
author_rank: Super Contributor
timestamp: February 03, 2022, 06:51:41 pm
content: Do the signal levels match? For example, if the 34401A transmits at 3.3V LVTTL and your receiver expects 5V TTL, then sending will work but receiving may not.Have you performed a loopback test on your device? If you connect Tx to Rx and type characters into PuTTY, do you see echos?

author: ogden
author_rank: Super Contributor
timestamp: February 03, 2022, 08:44:00 pm
content: First thing I would do - check cable, then rs232 of the computer, using loopback. After that I would look at the TX and RX using oscilloscope.

author: alm
author_rank: Super Contributor
timestamp: February 04, 2022, 11:28:20 am
content: Is talk only enabled in the I/O menu under HP-IB?

author: m k
author_rank: Super Contributor
timestamp: February 05, 2022, 08:21:04 pm
content: Quote from: fzabkar on February 03, 2022, 06:51:41 pmRS-port should be +/-.At least something below GND.

author: JOE007
author_rank: Newbie
timestamp: February 06, 2022, 08:54:37 am
content: Use cross cables

author: JOE007
author_rank: Newbie
timestamp: February 06, 2022, 09:03:39 am
content: Follow this link

author: Karel
author_rank: Super Contributor
timestamp: February 06, 2022, 09:11:22 am
content: According to the manual, the 34401A use hardware handshaking using DTR (pin 4) and DSR (pin 6).(Pin numbers are related to the DB9 connector.)If they are not connected or in the wrong state, the device will refuse to transmit characters.Most modern devices use RTS (pin 7) and CTS (pin 8 ) for hardware handshaking.Have a look at page 151 of the manual section: "DTR / DSR Handshake Protocol".Try using a fully wired (all pins connected) null-modem/cross cable:1 <-> 12 <-> 33 <-> 27 <-> 88 <-> 74 <-> 66 <-> 45 <-> 59 <-> 9Measure the levels on pin 4 and 6 before and after starting Putty.

author: m k
author_rank: Super Contributor
timestamp: February 06, 2022, 01:40:51 pm
content: Since commands are received much is ok.PuTTY default Flow control is Xon/Xoff so no problems there.You can also try None but it doesn't really change anything.From manual page 151 you'll see that usual simple cable looping 7-8 and 4-6 is no good.DTR is dropped when Talk starts and so send(DSR off) is also stopped.There are also RS-USB adapters without proper voltages.Since DMM is receiving it should see DSR on also but you should measure what happens in DMM's Tx line.If you can't check Tx line you can also check DMM side voltage levels.Other transmitters are DTR and RTS.There are On and Off voltages, what are they?From manual page 150(Adapter) you'll see that DMM's RTS is active.So you can ensure DMM's transmit by looping DMM's RTS and DSR.Adapter's RTS to DCD is modem stuff.Back in the day some terminals were picky and DCD on was needed.

author: Karel
author_rank: Super Contributor
timestamp: February 06, 2022, 02:05:53 pm
content: Quote from: m k on February 06, 2022, 01:40:51 pmNo, it's ok only for one direction. Flowcontrol is independent for RX and TX.Quote from: m k on February 06, 2022, 01:40:51 pmNo, if the DMM uses hardware flowcontrol, it continuously checks that pin if it's allowed to transmit.If the pc/terminal is setup for software flowcontrol or no flowcontrol, that pin is never set in the correct stateand the DMM will suspend TX forever (because it thinks the pc/terminal is not ready to receive).

author: m k
author_rank: Super Contributor
timestamp: February 06, 2022, 08:28:39 pm
content: There were a word "much."It includes things like speed and stuff.Read the whole text.Answer is in the next paragraph.