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Motorola 6845 Programmable?

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Chromedome45

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As title says is the 6845 CRT Controller programmable? I mean did Motorola set it up for specific customers? According to video modes the computer required? Reason for asking is my Model 4 NGA has a suspect CRTC. I tried I using a different one and no screen output. Was just wondering.
 
It SHOULD be programmed thru registers...
Perhaps the 4 is having touble reading/writing to the CRTC regs?
 
leeb said:
It SHOULD be programmed thru registers...
Perhaps the 4 is having touble reading/writing to the CRTC regs?
Click to expand...

Yes, if the 6845 CRTC is not initialized by the CPU writing to its registers with data, there will be no vertical and horizontal sync signals and hence a blank screen. Check that the CPU is running correctly. It appears it is getting lost before the I/O devices are initialized (which is very early on power up).
 
Chromedome45 said:
As title says is the 6845 CRT Controller programmable? I mean did Motorola set it up for specific customers? According to video modes the computer required? Reason for asking is my Model 4 NGA has a suspect CRTC. I tried I using a different one and no screen output. Was just wondering.
Click to expand...

If your question is whether the 6845 was somehow customised for specific machines (ie, contains custom firmware or logic), the answer is no. They do come in different flavours (1 or 2 Mhz, with or without Transparant Mode). But if you buy the right version from Utsource or someone like that, it should be a drop-in replacement.

The VDU board from the N8VEM project uses the 6845. They use the 2Mhz (or more) version which is required for 80 column output, and the version with the Transparent Mode. The quote below describes which 6845/6545s fit that bill. I believe (but am too incompetent to be sure!) that these variants have the superset of features to make them good bets:

SY6545A (Synertek, 2MHz), the UM6845EA (United Microelectronics, 2MHz), and the UM6845EB (United Microelectronics, 3 MHz).

Anyway, the Synertek data sheet has a good intro on the different versions if you want to be sure. They cost $4 or so, so the gamble of getting one from Utsource carries limited financial risk...

Cheers,

Oscar.
 
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My point to Frank was that any old 6845 in a given application may not work correctly. There are differing speed grades 6845=1 MHz, 68A45 = 1.5MHz, 68B45 = 2 MHz as well as the -1 versions that add bits in various registers for additional control over sync. There is the Hitachi version of the controller, which is very different in some aspects. I was a bit surprised when a plain-Jane 6845 would not work in my AT&T 6300--it required the Hitachi chip. One of these days, I'll figure out why.
 
The TRS-80 Model 4 does not use an MC6845, it uses an MC6835 or SY68045. Those are like an MC6845 but registers R0-R11 are read-only, with values programmed by a mask provided by the customer. (The mask actually specifies 24 bytes, two values for each register, and the system can choose which set to use at runtime by toggling the input to pin 3.)

So no, you can't replace one MC6835 with another unless they have the same mask ID (which is SC80756P for the 60 hz Motorola version of the Model 4 CRTC).

You can get a Model 4 to work with a generic MC6845 if you write your own code to initialize all the registers, and include that code on your boot floppy. An advantage to doing that is that you can get 80x25 text if you want (in addition to the Model 4 standard modes, 64x16 and 80x24).
 
Explains why the only one that would work was from my other model 4. Interesting. Thanks Petrofsky you confirmed my suspicion about being programmed. And apologies to the rest of the group thought it was a 6845 not a 68045/6835.

So Petrofsky do you know how to write this code?
 
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