brad@kontron.UUCP (Brad Yearwood) (04/25/85)
Yes, it is possible for software to fry hardware. I have seen a Compaq monitor destroyed by software run amok. The same thing should be possible on an IBM monochrome. If the 6845 CRT controller is grossly misprogrammed, either deliberately or by a wild jump into some part of the BIOS ROM, it seems possible that the horizontal or vertical drive outputs will be continually asserted. (See the cautionary note on page 1-115 of the IBM XT Technical Reference manual.) The horizontal drive circuit in the IBM monochrome monitor (and apparently also in the Compaq) is DC-coupled. A relatively small transistor is used to drive the primary of a transformer. If the horizontal drive is continually asserted, a DC current of about 300ma. will be passing through this tiny transistor, which amounts to about 4 watts with the 15V supply used in the monitor. (There is a 47ohm 5W resistor in the circuit. This is fine for limiting peak current in normal operation, but...) Typical maximum continuous power dissipation figures for transistors in small plastic packages are in the 300-650mw range. (The fried transistor I replaced in the Compaq was in a small plastic package.) Given a chain consisting of a 5W resistor, a transformer primary, and a tiny transistor, the weakest link will usually be the transistor. The funny smell is the plastic package burning. Damage to the transformer is not out of the question. Fortunately, the Compaq's transformer survived. The components are entirely adequate for the normal mode of operation. The transistor in question handles the peak current only for a narrow duty cycle, and is properly specified for that application. Unfortunately, the direct coupled input affords no protection against a continuously asserted horizontal drive.
hart@cp1.UUCP (Rod Hart) (04/28/85)
> Yes, it is possible for software to fry hardware. > I have seen a Compaq monitor destroyed by software run > amok. The same thing should be possible on an IBM monochrome. > > If the 6845 CRT controller is grossly misprogrammed, either > deliberately or by a wild jump into some part of the BIOS ROM, > it seems possible that the horizontal or vertical drive outputs > will be continually asserted. (See the cautionary note on page > 1-115 of the IBM XT Technical Reference manual.) > > The horizontal drive circuit in the IBM monochrome monitor (and > apparently also in the Compaq) is DC-coupled. A relatively small > transistor is used to drive the primary of a transformer. If the > horizontal drive is continually asserted, a DC current of about > 300ma. will be passing through this tiny transistor, which amounts > to about 4 watts with the 15V supply used in the monitor. (There > is a 47ohm 5W resistor in the circuit. This is fine for limiting > peak current in normal operation, but...) > > Typical maximum continuous power dissipation figures for transistors > in small plastic packages are in the 300-650mw range. (The fried > transistor I replaced in the Compaq was in a small plastic package.) > Given a chain consisting of a 5W resistor, a transformer primary, and > a tiny transistor, the weakest link will usually be the transistor. > The funny smell is the plastic package burning. Damage to the > transformer is not out of the question. Fortunately, the Compaq's > transformer survived. > > The components are entirely adequate for the normal mode of operation. > The transistor in question handles the peak current only for a narrow > duty cycle, and is properly specified for that application. Unfortunately, > the direct coupled input affords no protection against a continuously > asserted horizontal drive. Is there a possibility of this happening with the AT&T 6300? -- =========================================================================== Signed by: Rod Hart (WA3MEZ) Minicomputer Technical Support District Chesapeake & Potomac Tel. Co. A Bell Atlantic Company Silver Spring, Md. sabre!cp1!hart - gamma!cp1!hart - umcp-cs!cp1!hart - aplvax!cp1!hart ===========================================================================