wsdwgk@eutrc3.urc.tue.nl (g.v.kooij) (09/28/89)
Hello I want to program the 8250 in a PC clone to communicate with a MIDI device, which uses the standard MIDI baudrate of 31250 baud. Unfortunately, my docs say that you can only program it for 9600 (and slower), 19200 and 38400. Is it possible to do what I want ? Or should I buy additional hardware ?? greetings Guido
troy@mr_plod.cbme.unsw.oz (Troy Rollo) (10/03/89)
From article <927@eutrc3.urc.tue.nl>, by wsdwgk@eutrc3.urc.tue.nl (g.v.kooij): wsdwgk> Hello wsdwgk> I want to program the 8250 in a PC clone to communicate with a MIDI device, wsdwgk> which uses the standard MIDI baudrate of 31250 baud. Unfortunately, my docs wsdwgk> say that you can only program it for 9600 (and slower), 19200 and 38400. wsdwgk> Is it possible to do what I want ? Or should I buy additional hardware ?? Yes - you need to know a little about the operation of the UART chip in the COM ports. Firstly, we all know about the documentation for the serial ports - how INT 14H can initialise/operate both COM ports. Don't you believe it. It simply doesn't work. No explaination, no reason, nothing. It just doesn't work. In fact, as far as I know, no IBM communications software uses INT 14H to communicate with the COM ports. So you will need to get a full data sheet for the 8250 and use that. In particular, you will be interested in ports 3f8 and 3f9 when DLAB is 1. These are the baud rate divisor bytes. The procedure to store a value into these is: IN al, 3fbh OR al, 80h OUT 3fbh, al MOV al, low_byte OUT 3f8h, al MOV al, high_byte OUT 3f9h, al IN al, 3fbh AND al, 7fh OUT 3fbh, al The clock rate for the UART is 1.8432 MHz, implying that the value for the divisor should be 589, or 2 (high), 77 (low). I have a sneaking suspicion that this needs to be divided by 16... or perhaps even multiplied by 16..... try all of those and see. Now the problem with INT14? Well, I have had a guess as to what the problem is, but I haven't been able to test it. At boot time, COM interrupts are disabled, and from memory (when I traced INT14), it set up the serial port to use interrupts, which were simply being masked off by the PIC. You should be able to enable them with the following: cli mov al, 13h out 20h, al mov al, 8 out 21h, al mov al, 9 out 21h, al mov al, 0a4h out 21h, al sti Briefly, this reconfigures the PIC to let the normal interrupts (disk, keyboard and real time clock) through, as well as the two serial interrupts. If you have a hard disk, you will need to change 0a4h to 084h, which enables everything except for the printer, and the unused interrupt. Note that this disables the second PIC on an AT or better. If you are using interrupts from devices connected to this, you will need to go through a slightly longer procedure, which I don't have. ___________________________________________________________ troy@mr_plod.cbme.unsw.oz.au Make our greenies useful! The Resident Fascist Put them in the army!
scott@bbxsda.UUCP (Scott Amspoker) (10/04/89)
In article <446@usage.csd.unsw.oz> troy@mr_plod.cbme.unsw.oz writes: >From article <927@eutrc3.urc.tue.nl>, by wsdwgk@eutrc3.urc.tue.nl (g.v.kooij): >>[somebody wants to use com port for MIDI] >[advises that INT 14H doesn't work very well] It is true. There is no standard interrupt service of the com ports on a PC. If you intend to *really* use the com ports (especially for MIDI) you will have to write your own driver that includes interrupt handling (this shouldn't be a major task). I can't comment on the ability to run the com port at 31k baud but I remember reading an article a few years ago describing how to modify a standard PC communications adapter to support MIDI. It involved making a small mod to the card itself (breaking a connection and adding a jumper). This left me with the impression that you could not use the communications adapter in its normal state for MIDI. I'm sorry I can't tell you more. I would advise checking into this further. If all else fails then check out the Roland MPU-401. Not only does this guarantee you MIDI capability but there is plenty of existing MIDI software that supports the MPU-401. -- Scott Amspoker Basis International, Albuquerque, NM (505) 345-5232
clyde@hitech.ht.oz (Clyde Smith-Stubbs) (10/05/89)
From article <446@usage.csd.unsw.oz>, by troy@mr_plod.cbme.unsw.oz (Troy Rollo): > From article <927@eutrc3.urc.tue.nl>, by wsdwgk@eutrc3.urc.tue.nl (g.v.kooij): > wsdwgk> Hello > > wsdwgk> I want to program the 8250 in a PC clone to communicate with a MIDI device, > wsdwgk> which uses the standard MIDI baudrate of 31250 baud. Unfortunately, my docs > wsdwgk> say that you can only program it for 9600 (and slower), 19200 and 38400. > wsdwgk> Is it possible to do what I want ? Or should I buy additional hardware ?? > > The clock rate for the UART is 1.8432 MHz, implying that the value for the > divisor should be 589, or 2 (high), 77 (low). I have a sneaking suspicion that > this needs to be divided by 16... or perhaps even multiplied by 16..... try > all of those and see. Nope, 1.8432Mhz divided by 31250 is 59, not 589. And therein lies a problem. The actual division value you program in has to be Clock rate/baud rate/16 and 1843200 Hz/31250 bps/16 gives us a value of 3.686. There is no way to program fractional division ratios. That is why 38400 baud can be programmed but not 31250. 38400 requires a division ratio of 3, which is an integer. That is actually why the crystal frequency was chosen. To program 31250 you will need to change the crystal. You could use clock rate = baud rate*16*N where N is any integer (i.e. the division ratio you will end up with) So keeping it close to the original, clock rate = 31250 * 16 * 3 = 1.5Mhz In other words, change the crystal to 1.5MHz and program the baud rate division to 3 and you will get 31250 baud. Maybe this where 31250 came from? It obviously can be generated from clock rates which are fairly rounded numbers. ------------------------ Clyde Smith-Stubbs HI-TECH Software, P.O. Box 103, ALDERLEY, QLD, 4051, AUSTRALIA. ACSnet: clyde@hitech.ht.oz INTERNET: clyde@hitech.ht.oz.au PHONE: +61 7 300 5011 UUCP: uunet!hitech.ht.oz.au!clyde FAX: +61 7 300 5246 -- Clyde Smith-Stubbs HI-TECH Software, P.O. Box 103, ALDERLEY, QLD, 4051, AUSTRALIA. INTERNET: clyde@hitech.ht.oz.au PHONE: +61 7 300 5011 UUCP: uunet!hitech.ht.oz.au!clyde FAX: +61 7 300 5246