What causes no communication to the Data Link Connector?

I have a 91 chevy s10 and had a soft code come up (meaning check engine light came on for brief time then went off) so I tried to hook a OBD 1 scan tool up to the DLC to see what the code was and I get no communication. I've checked the ground on the DLC, ECM and PCM. There all good. I've also checked the resistance from the DLC ground to the ECM ground and have a resistance of 1.0 which is good. What is cause to computer not to communicate?