The IBM 5150 and 5160 mainboards are not designed to run at any other clock than 4,77 MHz. Because not only the processor clock, but also the CGA video timings, the RAM refresh timing, the system clock and the speaker frequency are derived from that clock, running the processor at a higher clock than 4.77MHz on these boards can only work with a processor replacement board that generates a dedicated processor clock. Modules like that exist - but if you already have an 8MHz Turbo XT, just use that machine when you need more power.

On the other hand, you can replace the 8088 by a NEC V20. Even if the V20 is clocked at the same frequency, 4.77MHz, it will usually run about 10% faster.

You can not get a higher processor speed by just inserting a "faster" processor. A 8088-1 or 8088-2 will be exactly as fast as the 5MHz chip that IBM originally used, until you provide a faster clock. The speed on the label of the processor is just the maximum speed you may use it at.

As always, if you want to know about old IBM stuff, -0° is a good reference. Here it tells you that there are 2 revisions of the XT mainboard, and the second revision allows 640KB onboard memory. Also, that page has a note that IBM authorized service technicians to add a bodge to upgrade the original board to 640KB.

The combination of an RCA connector and a 9-pin output is a good indicator for a CGA card. The CVBS output is always active, but it only ouputs NTSC color in video modes 1 (co40),3 (co80) and 4 (320x200x4). The RGBI connector has color also in modes 0,2 and 5, which are colorless on the CVBS output on purpose. Note that this is not an RF output, it is a baseband CVBS signal, you can feed it into an RF modulator or into a "video in" jack. Later TVs use a yellow-colored jack for CVBS video in.

mkarcher wrote on 2021-09-05, 21:49:

The IBM 5150 and 5160 mainboards are not designed to run at any other clock than 4,77 MHz. Because not only the processor clock, […]
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The IBM 5150 and 5160 mainboards are not designed to run at any other clock than 4,77 MHz. Because not only the processor clock, but also the CGA video timings, the RAM refresh timing, the system clock and the speaker frequency are derived from that clock, running the processor at a higher clock than 4.77MHz on these boards can only work with a processor replacement board that generates a dedicated processor clock. Modules like that exist - but if you already have an 8MHz Turbo XT, just use that machine when you need more power.

On the other hand, you can replace the 8088 by a NEC V20. Even if the V20 is clocked at the same frequency, 4.77MHz, it will usually run about 10% faster.

You can not get a higher processor speed by just inserting a "faster" processor. A 8088-1 or 8088-2 will be exactly as fast as the 5MHz chip that IBM originally used, until you provide a faster clock. The speed on the label of the processor is just the maximum speed you may use it at.

As always, if you want to know about old IBM stuff, -0° is a good reference. Here it tells you that there are 2 revisions of the XT mainboard, and the second revision allows 640KB onboard memory. Also, that page has a note that IBM authorized service technicians to add a bodge to upgrade the original board to 640KB.

The combination of an RCA connector and a 9-pin output is a good indicator for a CGA card. The CVBS output is always active, but it only ouputs NTSC color in video modes 1 (co40),3 (co80) and 4 (320x200x4). The RGBI connector has color also in modes 0,2 and 5, which are colorless on the CVBS output on purpose. Note that this is not an RF output, it is a baseband CVBS signal, you can feed it into an RF modulator or into a "video in" jack. Later TVs use a yellow-colored jack for CVBS video in.

Thanks a lot for these answers.
I just read a lot more on forums and google. Could you answer these 3 questions too guys thanks a lot!

1. Just to be sure at 100% to understand correctly, this seller with the nec v20 at 16mhz said it works in a 8088 ibm, but it wont run at 16 mhz ? If i understand you correctly, it will still run at 4.77mhz ? right ? see below, thanks.
( in his xt clone computer benchmark result are good as a 286 at 8mhz, but i dont know at which speed his cpu runs)

https://www.ebay.ca/itm/224177951505?_trkparm … 7Cclp%3A2334524

2. Edit: I have another question. On my xt clone 8mhz turbo motherboard with a 8088-2. Could i use a 8088-1 10mhz and runs it at 10mhz ? or it will only runs at 8mhz ? Thanks

3. Edit: I just read more. Its seems a 8mhz turbo board runs cpu at 7.16mhz, is it right ?

Thanks for answer again, it would be very appreciated.

( I just read a lot about all of this, crystal oscillator in mhz, isa speed and overclock.. i think i get it now, i understand most of it. But i would still appreciate the answer to my last 3 questions. This way everything would be understand correctly.)

The PC-Sprint is a good option for overclocking the 5150/60

renejr902 wrote on 2021-09-05, 22:31:

1. Just to be sure at 100% to understand correctly, this seller with the nec v20 at 16mhz said it works in a 8088 ibm, but it wont run at 16 mhz ? If i understand you correctly, it will still run at 4.77mhz ? right ? see below, thanks.
( in his xt clone computer benchmark result are good as a 286 at 8mhz, but i dont know at which speed his cpu runs)

https://www.ebay.ca/itm/224177951505?_trkparm … 7Cclp%3A2334524

I just looked at the eBay offer as it is at the moment. The offer is quite clear that the V20 will run at "up to" 16MHz, but also work at lower speeds. The benchmark screenshots show the performance of a V20 at 4.77MHz, I.e. 1.33 times the PC performance in the MIPS tool and 1.13 times the PC performance in checkit. The processor would be way faster if it were clocked at 16MHz. A 286 at 8MHz should perform considerably better, I expect it to be around 2.0 times PC performance.

renejr902 wrote on 2021-09-05, 22:31:

2. Edit: I have another question. On my xt clone 8mhz turbo motherboard with a 8088-2. Could i use a 8088-1 10mhz and runs it at 10mhz ? or it will only runs at 8mhz ? Thanks

The same theme applies here, too. If you just change the processor, the clock rate doesn't change. If your Turbo XT is running at 8.00 MHz, it is very likely that there is a 24.000 silver box, a quartz oscillator, installed on the mainboard. You would need to replace that oscillator by a 30.000 MHz oscillator to run the board at 10MHz. This 24MHz or 30MHz oscillator only drives the processor (unlike in the IBM PC, where one oscillator drives everything), so swapping this oscillator has no unwanted side effects.

Of course, a 10MHz processor requires the board to reply faster than a 8MHz processor. It is unknown whether your board is fast enough to support a 10MHz processor. Running a 8MHz board at 10MHz us overclocking parts of the board, and thus may fail. I have a genuine 10MHz Turbo XT board, and noticed that this board adds an extra wait state to ISA cycles at 10 MHz. This is done to improve ISA compatibility. Chances are that 8MHz boards do not yet add the extra wait state - and if that is the case ISA compatibility might be poor at 10 MHz.

renejr902 wrote on 2021-09-05, 22:31:

3. Edit: I just read more. Its seems a 8mhz turbo board runs cpu at 7.16mhz, is it right ?

No. There are two kinds of Turbo XT implementations. The classic one is a bog standard XT board where you just switch the processor clock from 4.77MHz (generated as 14.318/3) to a higher dedicated clock generated from a different source, usually a crystal oscillator can. This implementation needs minimal changes compared to the original IBM PC design. In this design, you usually get 6, 8 or 10 MHz exactly.

On the other hand, there are enhanced XT board designs that allow clocking the processor at 14.318/2 instead of 14.318/3 without requiring a dedicated processor clock generator. This design replaces the original Intel 8284 clock/ready generator chip by a custom implementation. On those boards, the turbo speed will be 7.16MHz.

If I remember correctly, you were concerned about timing compatibility with the 4.77MHz IBM PC in the thread about your Turbo XT. Now that you got the "real thing", not only the 5160 motherboard, but also likely the original CGA, you have a system that is exactly the original speed. If having such a thing is important to you, do not overclock the 5160, do not install a V20, do not install a different graphics card and do not install a PC-sprint board. Instead, do the tuning experiments on the Turbo XT instead.

renejr902 wrote on 2021-09-05, 21:33:

2. Mine has 640kb on picture boot screen. So does it include a upgrade isa ram memory card ? or it could be on the motherboard ?

I have 2 5160, the earlier version has a max of 256kb on motherboard and my later version has a max of 640kb on motherboard

3. I think it comes with a cga card, could i use the cga rca output when i boot the computer ? ( i dont have a cga monitor) Otherwise i will have to use my isa vga cards, but i would like to use the cga rca output. I think its a cga card because it has a rca output and a 9 pins output on the same card. No other connector. Is it for sure a cga card ? Is the rca output would be in colors ? I dont want it in white and black, some people on forum have that sometime instead of color output ?

CGA is the standard card that came with the 5160 so its likely it has a composite output that should be color.

4. Does my DTK keyboard with a switch for AT or XT work with it ?

Should work fine I have a couple of AT/XT switchable keyboards and they all work on my 5160s

mkarcher wrote on 2021-09-06, 07:32:

If I remember correctly, you were concerned about timing compatibility with the 4.77MHz IBM PC in the thread about your Turbo XT. Now that you got the "real thing", not only the 5160 motherboard, but also likely the original CGA, you have a system that is exactly the original speed. If having such a thing is important to you, do not overclock the 5160, do not install a V20, do not install a different graphics card and do not install a PC-sprint board. Instead, do the tuning experiments on the Turbo XT instead.

Thanks a lot for all these answers.Yes i will do exactly like you just told me. Its a great idea !

Grem Five wrote on 2021-09-07, 00:47:

I have 2 5160, the earlier version has a max of 256kb on motherboard and my later version has a max of 640kb on motherboard […]
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renejr902 wrote on 2021-09-05, 21:33:

2. Mine has 640kb on picture boot screen. So does it include a upgrade isa ram memory card ? or it could be on the motherboard ?

I have 2 5160, the earlier version has a max of 256kb on motherboard and my later version has a max of 640kb on motherboard

3. I think it comes with a cga card, could i use the cga rca output when i boot the computer ? ( i dont have a cga monitor) Otherwise i will have to use my isa vga cards, but i would like to use the cga rca output. I think its a cga card because it has a rca output and a 9 pins output on the same card. No other connector. Is it for sure a cga card ? Is the rca output would be in colors ? I dont want it in white and black, some people on forum have that sometime instead of color output ?

CGA is the standard card that came with the 5160 so its likely it has a composite output that should be color.

4. Does my DTK keyboard with a switch for AT or XT work with it ?

Should work fine I have a couple of AT/XT switchable keyboards and they all work on my 5160s

Thanks a lot for these answers, its really appreciated.

maxtherabbit wrote on 2021-09-06, 01:03:

The PC-Sprint is a good option for overclocking the 5150/60

Thanks for your answer, i read a lot about it, its interesting.