interface between fpga and CGA-CRT tube driver - Page 1
Forum section: electronics / general

author: legacy
author_rank: Super Contributor
timestamp: August 03, 2018, 07:25:25 am
content: so I have recently purchased a new CRT amber tube, it's 1 bit of color, and it's CGA timing. The diver is powered at 12V, and it offers a CGA interface that is TTL/5V.I am willing to adapt the VDU module I have implemented for VGA LCD. Modifications will impact on the PHY sub-module, I need to change the polarity of the sync signal as well as the timing), and signals need to be TTL/5V, thus I also wonder if it's safe to directly couple it to a fpga's pins through a level shifter, or if I'd better pass through a safer way.Hints?p.s.I am willing to build a wood box for the glass tube, I will take a few pics once completed, the whole equipment, tube+fpga(+softcore implemented into the fpga), will serve asVT100 terminal

author: legacy
author_rank: Super Contributor
timestamp: August 03, 2018, 02:42:04 pm
content: in the meanwhile, as a temporary solution to go ahead with the implementation, a 5V-tolerant XC9572 CPLD has been put between the FPGA and the TTL interface of the Video driver of the CGA tube, used as a buffer since its IO-gate-set is more strong than the FPGA

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2018, 04:09:44 pm
content: I have a similar monitor with a green monochrome tube, it works perfectly driven directly from an FPGA, changing the sync polarity is trivial by modifying the HDL.

author: legacy
author_rank: Super Contributor
timestamp: August 03, 2018, 04:55:04 pm
content: Quote from: james_s on August 03, 2018, 04:09:44 pmaren't you a bit worried about the fragility of the FPGA IO? The tube works at 50Kvolt, and it's not exactly well shielded.Quote from: james_s on August 03, 2018, 04:09:44 pmMy project started with a VGA VDU, the tube-diver is CGA, it also requires to adapt the pixel frequency, as well as the polarity of the sync. But this is not a problem.What I am worried is about the electrical interface, due to the CRT tube. The used CPLD at the interface costs a fraction vs the FPGA.

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2018, 05:16:51 pm
content: I'm not worried at all, I'm not driving a bare CRT directly from the FPGA, it's an open frame monitor with TTL inputs designed to be driven directly by TTL logic. If you do have some kind of bizarre fault that results in high voltage on the video input pins, a CPLD is not going to block that. If you look inside the monitor you will find spark gaps, occasionally NE-2 type lamps used to prevent a fault condition inside the CRT from feeding HV back into the circuitry.Not that it matters here, but 50kV is WAY high, most small CRTs run about 8-15kV anode voltage, 25kV is the legal limit for consumer gear.

author: legacy
author_rank: Super Contributor
timestamp: August 03, 2018, 07:53:41 pm
content: what about the current required by the TTL circuit? Being an old-TTL circuit the out current might need to be higher than the current that the FPGA can provide. In this case, the CPLD has higher current.p.s.yes, it's written 15KV on the tube. I misread the label, the "1" is not so clear, and it looked there was a zero between "5" and "Kv"

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2018, 09:54:04 pm
content: You might need a level shifter or buffer in some cases, particularly if the monitor happens to have 5V pullups on any lines, however in practice I have never found this to be the case. FPGA pins, at least on the Altera and Xilinx parts I'm using can supply significant current, plenty to drive an LED directly. TTL monitors normally do not require a strong drive. YMMV but I have done this lots of times and never had any problem at all.

author: niladherbert
author_rank: Contributor
timestamp: August 07, 2018, 07:47:08 pm
content: Hey, do you have any idea where I can get documents for the CGA interface? (old IBM PC-compatible to sort out with a monitor)

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2018, 07:50:26 pm
content: I don't remember where I found it but the specs are straightforward. The scan rates are the same as NTSC TV, signals use 5V TTL levels, my recollection is that it uses positive sync pulses but you should double check that. In addition to the three color lines there is an intensity line allowing a total of 16 colors, one of those being black.

author: james_s
author_rank: Super Contributor
timestamp: August 08, 2018, 12:16:48 am
content: Must not have looked very hard.http://nemesis.lonestar.org/reference/video/cga.htmlhttps://timetoexplore.net/blog/video-timings-vga-720p-1080phttp://www.epanorama.net/documents/pc/vga_timing.htmlhttp://minuszerodegrees.net/mda_cga_ega/mda_cga_ega.htmhttp://www.zxdesign.info/horiztiming.shtml