The pull-up resistor (to VCC) should be 3.1KOhm (+/-5%) while the pull-down (to signal ground) should be 6.2 KOhm. Check both the NDAC and the NRFD. **vp PS Of course now that I checked the circiut diagram of the 82937A HP-IB Interface for the HP Series 80 computers, I see that they have a 75162N bi-directional bus tranceiver connected directly to the NRFD line (no resistors).