superfury wrote on 2025-12-26, 15:05:
That fourth brightness is documented on the wiki page, though: https://en.wikipedia.org/wiki/IBM_Monochrome_Display_Adapter
Under Capabilities.
In my oppinion, the claim that the levels are (exactly) 33%, 66% and 100% should not be on Wikipedia due to WP:NOR. Typical monochrome monitors have user-adjustable brightness and contrast controls, and if I remember correctly, "brightness" directly sets the brightness level of "VIDEO=1/INTENSITY=1" (it defines "100%"), and the contrast control allows an arbitrary setting of "normal video" between 0% and 100%. So adjusting it to 66% is possible, but clearly not given in any way.
superfury wrote on 2025-12-26, 15:05:That does make me question: what happens on a genuine IBM MDA monitor?
Page D-24 of the first edition of the IBM PC Technical Reference Manual has the schematics of the 5151 (the "genuine IBM MDA monitor"). Video input is handled by the 7406 open-collector hex inverter IC 201. R214/D203 are a shunt regulator that provide 5V Vcc to that chip, R204 and R202, while (A) is connected to the stabilized 15V supply. The 7406 is capable of handling up to 15V at its open-collector outputs, so you don't need to worry whether the 15V fries the chip.
Furthermore, the output of the video input circuit is the right end of R207, which enters the cathode drive amplifier at the point labeled "V1", with ground being connected to "E1". The first active element in the cathode drive amplifier is TR19, a bipolar transistor. This transistor will be "completely off" as long as the base voltage is below 0.6V, and during normal operation, the base voltage will be around 0.7V. I'm unable to explain what exactly happens in that circuit in its active region, but basically, higher voltage at V1 means higher current through R208, which means more current through TR19, which will pull more current through TR20, and thus considerably more current through R209, which counteracts the voltage at V1. This counteraction is used to generate a stable amplification via TR19, independent of component variations and the temperature of TR19. The input circuit of the cathode drive amplifier seems to be cascode circuit. If you happen to be interested in the analog design, this keyword enables you to dig deeper. The main point is that anything below 0.6V at E1 is treated the same way, as "completely off". The other end of the operating region is set by the 6.8V at the base of TR20 and/or the voltage drop across R211, but it is likely higher than +5V at the input, so it will not be relevant for the further discussion.
Now let's look at how the VIDEO signal is processed. The VIDEO signal first enters a discrete inverting amplifier built from R201 and TR18. The capacitor in parallel to the base resistor enhances response to high frequencies and thus will increase image sharpness. I assume this provides better high-frequency response to the VIDEO signal than to the INTESITY signal, which is a sign that you likely get sub-par results assuming you want to use the INTENSITY signal per-pixel (which neither the MDA nor the Hercules card does). The externally inverted VIDEO signal is then sent through the inverter 9->8 of IC201, which will pull its output to ground if VIDEO is low. The SN7406 is quite good at pulling its output low (I didn't bother to check the data sheet right now, but I know its superior drive capability is the reason the AT uses the 7406 over the 7405 in its keyboard interface), so the cathode of D202 will be close to ground (0.3V to 0.5V), which is below the operating region of the cathode drive amplifier. So if VIDEO is low, the image is black, no matter what level the INTENSITY signal provides.
Only if VIDEO is high, the intensity signal gets relevant. It is connected to the input named "DUAL", which is inverted twice using the 7406 chip. The first time using the inverter 1->2 with a 5V pull-up R204, and a second time using the inverter 3->4. That inverter pulls pin 4 to ground if INTENSITY is low, and leaved pin 4 "open circuit" if INTENSiTY is high. If pin 4 is open circuit (high intensity), the circuit from +5V via R205, D201, the contrast pot and R206 to +15V does not conduct any current, because the diode D201 is obviously reverse biased. The circuit thus behaves "as if" D201, the contrast pot and R206 are removed from the circuit. This means all current through R205 continues to flow into the cathode drive amplifier (if VIDEO is high). On the other hand, if intensity is low, there is current flow via R206 (1K) and the contrast pot (500 ohms) into pin 4 of the inverter. Assuming no wiper current (which is not exactly true, but will do for simplicity right now), the wiper voltage of the contrast pot ranges between 0V and 5V if "low intensity" is requested. With a wiper voltage above 4.3V, D201 is still revese biased, so there is no response to the intensity signal if the wiper is close to the +5V end, i.e. there will be no contrast. On the other end of the contrast range, the wiper is directly connected to pin 4 of the inverter and pulled that hard to ground that the voltage at the cathode of D201, which is mirrored at the cathode of D202, is insufficient to turn on the cathode amplifier, making the "low intensity" signal completely black.
So, in summary, the circuit confirms my experience with monochrome monitors: You set the overall brightness (directly used for high-intensity video) using the brightness pot (which is in the CRT circuit), and you can then adjust the "normal intensity" brightness using the contrast pot, with an adjustment range including "normal video is invisible" to "normal video is as bright as high intensity" (both extremes are typically useless). The circuit of the 5151 does not allow a fourth shade of green in "intensity only" mode, because pin 8 of the inverter will pull all current that could enter the cathode amplifier if VIDEO is low, so VIDEO low directly implies "black" (unless you yank up the brightness to an insane level that makes even black light up the tube).
