Gary C
Veteran Member
A volunteer has brought in his Siemens PLC programmer which has suddenly decided to show a filled and corrupt screen.
Its almost as if 99% of all the character bits that should be in black are in green.
Machine seems to operate fine apart from the video. Pulled out the video board and pleased to find a simple implementation of a 6845 with a character ROM. Poking about the ROM address line from RA0 is bad. Its showing a trace but its only about 0.2V with an offset of about 4.5V so its never going to work.
Tried swapping the 6845 and removing the EPROM and it makes no difference. What is slightly odd, is that the output goes to one of the EPROM address pins and two 74 logic gates, a 74S86 and a 74LS74. The data sheet states that these outputs are capable of driving one TTL load and 30pf though.
Must have worked. Will replace and see whats pulling it up.
Its almost as if 99% of all the character bits that should be in black are in green.
Machine seems to operate fine apart from the video. Pulled out the video board and pleased to find a simple implementation of a 6845 with a character ROM. Poking about the ROM address line from RA0 is bad. Its showing a trace but its only about 0.2V with an offset of about 4.5V so its never going to work.
Tried swapping the 6845 and removing the EPROM and it makes no difference. What is slightly odd, is that the output goes to one of the EPROM address pins and two 74 logic gates, a 74S86 and a 74LS74. The data sheet states that these outputs are capable of driving one TTL load and 30pf though.
Must have worked. Will replace and see whats pulling it up.