Sunday, July 22, 2018

Ten Advantages of an IBM PC/XT over an IBM PC

If you are looking for an early PC-compatible "8-bit system", the IBM PC and the IBM PC/XT are good choices.  They sold well, are built to last and "just work".  But while you may want to get one of each, if money or space is tight, I would suggest trying to find an IBM PC/XT over the IBM PC.  Some reasons why are as follows :

1.  The IBM PC/XT's Mainboard Fits into any AT Case

If you buy an IBM PC motherboard, the only enclosure that motherboard will fit into is the one IBM designed for it or a clone enclosure.  Or the one you build yourself.  With a PC/XT motherboard, you can fit that into any AT form factor case.  So if you want a PC/XT Tower, you can have it.  Moreover, most tower cases and many aftermarket desktop cases can support eight full-length cards whereas the original IBM PC/XT's case can only support six full length cards. 

2.  The IBM PC/XT has More Expansion Slots than an IBM PC

8 > 5.  While both machines will have their slots filled with things like video adapters and drive controllers, the PC/XT does not need to waste a slot on adding conventional memory.  Although PC/XT Slot 8 can only be used by a small subset of ISA cards, an IBM Asynchronous Communications card is easy enough to find and almost always comes in the Slot 8 supporting variety.

3.  The IBM PC/XT BIOS can be Easily Upgraded

If your XT motherboard came with a 1982-dated BIOS and you wish to upgrade to the last 1986-dated BIOS for its newer features, it is as easy as burning a pair of 32KB EPROMs with the code and swapping the chips.  See here for more information : The IBM PC never received a BIOS update after 1982 and lacks the support for the new features of the 1986 XT BIOSes.

4.  The IBM PC/XT's RAM capacity can be Easily Upgraded and Saves a Slot

All IBM PC/XT motherboards can be upgraded to the maximum 640KB of Conventional Memory supported by the PCs and XTs.  All you need are the RAM chips. a standard multiplexer chip and a bit of solder and wire :

5.  All Four IBM PC/XT Banks of RAM are Socketed

If one of your RAM chips goes bad in their first bank of a IBM PC, you will need to desolder it to fix the problem.  Moveover, a dead DRAM chip in the first bank means that the system will appear dead.  You might have to desolder all nine chips until you find the culprit.  All banks of RAM chips in an IBM PC/XT are socketed for easy replacement.

6.  The PC/XT Works with all IBM-Manufactured Model M AT & PS/2 Keyboards

IBM released a LED-less 101-key Model M XT (1390120) keyboard late in the life of the IBM PC/XT.  The IBM PC/XT can also use keyboards released alongside the late model IBM PC ATs, the XT/286 and the PS/2s.  Compatibility was broken sometime after Lexmark took over Model M manufacture in 1992, but that still gives you six solid years of keyboards that can be good.  Compatibility between the Model Ms and the IBM PC is spotty, even with the keyboards IBM manufactured.  You will need a 1986 BIOS, but you can still use an 83-key XT keyboard with the 1986 BIOSes.  The keyboard LEDs will not work with the PC/XT and a Model M.

7.  The PC/XT Shows a RAM Count on Bootup

When an IBM PC boots, all you see is flashing cursor and then a single beep if all is well.  The IBM PC/XT will show a RAM count and when down then beeps.  The count eliminates guessing about how much RAM is installed in the system.  The extra display is helpful to new users to show that their computer is doing something while booting, whereas the older machine may confuse the new user into thinking it is faulty in some way.

8.  The PC/XT Autodetects the Amount of RAM

The IBM PC had a bank of dipswitches dedicated solely to indicating the amount of RAM installed in the system, including that included on expansion cards.  The PC/XT autodetects all the RAM from 0-640KB during the boot process, ditching the RAM-configuration bank of dipswitches.    Even though installing RAM on an IBM PC is a "set and forget" process, if you remove a RAM expansion card or accidentally bump into the dipswitches, you may be in for a memory error or a hang on your next boot.

9.  The PC/XT Power Supply has a far higher Wattage rating

The IBM PC came with a 63.5W power supply, the IBM PC/XT came with a 130W power supply.  The IBM PC's power supply was too weak to support the hard drives of its day reliably, a problem the IBM PC/XT did not have.  If you install the PC/XT in another case, you can enjoy the benefits of an even larger power supply.

10.  There is a Portable version of the PC/XT

The IBM PC Portable 5155 uses the PC/XT motherboard, so if you have a Portable PC, you can enjoy all the advantages described above with some caveats.  Only two of the Portable PC's slots can accommodate full length cards, the power supply is only 114W (but given you can fit only two drives in it, this isn't a big issue) and using a Model M keyboard will require you to construct an RJ11 to DIN5 adapter or cut some plastic off the back to get at the DIN port on the motherboard.

11.  Miscellaneous Minor Benefits

Among the minor benefits of the late-BIOS PC/XTs is support for 720KB floppies in the BIOS.  This means that you can format a disk with 80 cylinders in PC-DOS or MS-DOS without needing a config.sys line to tell DOS that the BIOS does not count up to 80 cylinders.  IBM PCs will format 720KB disks as 360KB disks without a config.sys line.  The BIOS also supports 1.2MB disks but the IBM Diskette Drive Adapter does not support 1.2MB drives.

The 1986 PC/XT BIOS comes with a BIOS routine for reading the joystick.  Games used their own routines because this routine was not included in earlier BIOSes, but this may be beneficial to programmers needing a routine that "just works".

The I/O range available to expansion cards in an IBM PC/XT is 100-3FF whereas IBM PCs only get 200-3FF.  Not many cards used ports in the 100-1FF range for this reason.  Moreover, this limitation should be limited to reading from the ports, not writing to the ports.

The IBM PC case can only mount disk drives on their left or right side, the IBM PC/XT case has a well cut into the bottom designed to secure drives from their bottom as well.

IBM PC/XTs came with full height and half height drives during their marketing life, the latter came as a pair of half height drives using a special mounting bracket.  IBM PCs only were supplied with full height drives.

The IBM PC/XT supports the IBM Professional Graphics Controller, the IBM PC does not (due to expansion card spacing, the PGC requires two adjacent expansion slots)

The IBM PC with the 1982 BIOS requires all four banks of RAM to be populated, the PC/XT requires only one bank.

The IBM PCs with the 1981 BIOSes only recognize up to 544KB of RAM and do not scan expansion slots for bootable ROMs (necessary for hard drives, EGA and VGA cards)

The early IBM PCs came with 16-64KB motherboards and single-sided disk drives.

Due to the standard EPROM pinout, "no-slot clocks" can work with the PC/XT.

The IBM PC/XT's motherboard was widely cloned, finding something that works identically or nearly so (no ROM BASIC unless you add it) to the real IBM machine is quite possible. 

The IBM PC/XT was originally sold with a hard drive and a serial card (later models had them as an option), so if you come across an original system, you could get these included. 

12.  Disadvantages of using an IBM PC/XT over an IBM PC

May require one floppy disk drive connected to boot or to avoid an error message.

Does not have a cassette interface for saving and loading in Cassette BASIC.

Uses a PROM for memory addressing, which makes it slightly more difficult to replace if the PROM goes bad

Technically the 5161 Expansion Unit is not supported with the 1986 BIOSes, but it still should work

1986 BIOSes has issues with some 3rd party diskette controllers booting

A few obscure games will not work in an PC/XT or maybe a late model PC/XT due to changes in the BIOS.

The IBM PC was almost always cheaper than the IBM PC/XT and were sold for a longer period of time, so IBM sold more PCs and they can be a little easier to find than PC/XTs.

Finally, the IBM PC came first!


Jonathan said...

Is the absence of a cassette port a real disadvantage? Was it usable for any other purpose than saving/loading games from BASIC? (e.g. in the TRS-80 computers, it could be used for sound output)

Anonymous said...

There is the original release of the IBM PC port of "The Desk Organizer" which used a dongle that was inserted in the cassette port. See page 247 of July 1983 PC Magazine so you know I am not making it up.

Couple of other programs like the IBM Diagnostic cassette version and I think one disk based game could save to cassette plus the IBM Music Feature Card covered in a previous post.