Trouble with RS-232/HP-IL Interface (HP 82164A)


thanks to the help of forum members i was able to solve a problem with my HP clock/HP-IL/HPIB setup. i could need some advice again. this time it's about the infamous RS-232/HP-IL interface (HP 82164A).

i'd like to connect my garmin gps (gpsmap 60cs) to the interface and read NMEA data into my HP 71B, following the setup egan ford suggests (link). the gps sources the data correctly. i can connect the gps to my mac via a serial/usb adapter and monitor the NMEA data at 4800 baud, 8/1 bit, XON/XOFF, no parity. that part seems to work properly.

i am also able to set the control and character registers of the interface, either by DDL or REMOTE, and read them. the status bytes are a bit strange, because byte 4 is 11110000 or 01110000, although it should be 00000000 according to the manual, as this byte is "reserved for future use". bytes 1 to 3 seem to be fine.

the internal jumper is set to DTE.

but when i try to read the NMEA data from the gps, i don't get a byte of data into the register (ENTER USING "#,nA";G$ with FLAG -23 clear or just ENTER;G$ with FLAG -23 set). with flag -23 clear the T/R light is permanently on, with the controller sending one SDA after the other, or flashes up once with FLAG -23 set.

when i disconnect the gps from the interface the result is the very same. it seems as if no data is sent from the RS-232 side to the HP-IL side and vice versa.

am i facing a hardware problem here, or did i just miss something?

thank you very much for your help!

edited to add info about internal jumper

Edited: 6 Jan 2013, 1:10 p.m.


You should test the HP-IL/RS232C interface with your Mac first to make sure it works. If you have a test plug (with two DB-25 ports and a bunch of LEDs) you can test if the wiring is correct.


You should test the HP-IL/RS232C interface with your Mac first to make sure it works. If you have a test plug (with two DB-25 ports and a bunch of LEDs) you can test if the wiring is correct.

thank you for the advice! i know where i can get such a cable tester quickly. i only hope that i am able to understand the results... more reading :-)



I don't use Garmin GPS but I advise you to start without using XON/XOFF.


unfortunately i only found a loopback tester. when i connect it to a 24 to 9 adapter, and the adapter to my gps, the RD3/TD2 led is green with no connection type chosen and begins to flash green/red when i choose NMEA IN/NMEA OUT.

when i connect the loopback tester to the interface plug (jumper set to DTE) the DTR20/DSR6 and the CTS5/RTS4 leds are red, the RD3/TD2 led is green. it does not start flashing when i send/request data to/from the interface.

all leds are green when i push the reset-button on the interface.

when i set the jumper to DCE, the DTR20/DSR6 is red and the RD3/TD2 led is green. again, no flashing of the RD3/TD2 led when i send something to the interface.

is there any further testing needed or does the behaviour described above mean that there is a hardware problem?




The problem with a loopback tester is that you cannot identify the sending pin because it's directly connected to the receiving pin. With a multimeter you can try to measure the voltage between pin 2 and pin 7. A DTE should show something in the range of -10V.

IIRC, a DTE uses pins 2 (TxD), 4 (CTS) and 20 (DSR) as outputs. Pins 6(DSR) and 8 (DCD) are inputs.


oh dear, RS-232 seems to be a synonym for "plug and pray"...

however, i followed your advice and measured the voltage of the pins, first of the db25 on the interface: about -8.3V on pin 2 (TXD?), about +8.7V on pins 4 (RTS?) and 20 (DTR?). looks good to me.

then i connected the db25 to db9 adapter to the interface: -8.3V on pin 3, 8.7V on pins 4 and 7. looks good again, at least to me...

now the voltage on the female db9, connected to the gps: about -5.3V on pin 2.

again i have the impression that everything is fine, output from interface is on pin 3 (db9), output from the gps on pin 2 (db9).

still, there is no flow of data between the rs-232 side and the hp-il side of the interface.

i'll try some other configurations of the control- and character registers, although i do not expect too much. and then i'll probably try to get a second RS-232 interface... oh dear... :-)

thanks again,


edited: typo

Edited: 9 Jan 2013, 3:59 p.m.


Hans, that looks fine. Now it's time to check the transmission parameters (speed, word size and parity). This can be done by connecting to a PC or Mac and starting a serial terminal application. Then make the interface send something and check the results in the terminal window.


It has been 25 years since I used the 82164A, so my memory is rusty.

But shouldn't the 82164A be set up as DCE in this case? I thought that DCE was for the device playing the role of 'computer', and DTE for the device that was playing the role of 'terminal'. In the test with the Mac, wouldn't the Mac be set up as DCE? In which case, the Garmin would be DTE? Requiring then that the 82164 be set up as DCE?


Dan, I doubt it. To my understanding, computers are "terminals" while modems are "communication" equippment, hence the nomination as DTE and DCE. Connecting a DTE to a DCE requires a 1:1 connection. Connecting two DTEs is possible with a cross over cable.

A DTE sends on pin 2 (of a DB25 plug) and receives on pin 3. At the DCE side, pin 2 is an input and pin 3 is an output. Beware: The DB9 connector has pin 2 and 3 reversed compared to the DB25 connector.

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