IBM PC XT/370 Troubleshooting Guide

The XT/370 card pair, with the PC/370-P processor card on top, and IBM's Diagnostics/370 Personality diskette (6361927, version 1.03). Photo: Mike Ross, Wikimedia Commons, CC BY-SA 3.0.

The IBM PC XT/370 is a 5160 PC XT with two System/370 adapter cards: the Processor card (PC/370-P) in expansion slot 4 and the 512 KB Memory card (PC/370-M) in expansion slot 3. Its faults fall into three groups. XT host faults are reported by the normal XT power-on self test (POST). Faults in the 370 processor and the card memory it uses are reported by VM/PC when it starts or while it runs. Host-link faults are on the 3270 emulation adapter and its coaxial cable. For faults that are not specific to the XT/370, see the IBM PC XT Troubleshooting Guide.

Documents

• IBM Personal Computer Family Service Information Manual (SA38-0037-00). Chapter 4 has IBM's diagnostic tips for all 5150 and 5160-based systems, which chapter 6 names as the diagnostic section for the 5160 (p. 6-3). Pages 6-17 to 6-19 describe the XT/370.
• IBM Virtual Machine/Personal Computer User's Guide, first edition, December 1984. Pages 4-36 to 4-42 list VM/PC's startup failures and what to do about them; chapter 8 lists its messages.
• IBM Personal Computer XT/370 Guide to Operations. VM/PC's error messages send the user to its "Diagnostic Testing" section for the 370 hardware (VM/PC User's Guide p. 4-37). IBM supplied the diagnostics on a Diagnostics/370 Personality diskette, part 6361927, shown in the photo above with the cards.

Where to start

XT/370 symptoms

Symptom	Probable cause	Action
No cursor, no beep; fan may still turn	Shorted tantalum capacitor on the system board or a card; supply fault	Follow Dead machine below
Numeric error code during POST	XT host fault	Look up the code group below and in the IBM PC XT Troubleshooting Guide
DOS starts, but VMPC reports STORAGE ERROR (RC = 2670), PROCESSING UNIT ERROR (RC = 2680) or HARDWARE ERROR (RC = 2681)	370 card fault	Press F1 to retry, then run the XT/370 diagnostics; see VM/PC startup failures
VM/PC stops with IRRECOVERABLE MACHINE CHECK (DMK008T)	Error in the 370 processor with no recovery	Run the XT/370 diagnostics
The X (System Unavailable) indicator on the 25th line stays on	Host, link or local hardware	See Host connection
A diskette written on an AT/370 cannot be read	Not a fault: the XT/370 supports 360 KB diskettes only	Write the diskette on the XT/370 or a 360 KB drive

XT host faults

POST

POST runs each time the system unit is switched on and takes 13 to 90 seconds, depending on the options installed. A cursor appears after about 4 seconds, the memory count in the top left corner rises in 64 KB steps while memory is tested, and one short beep marks a successful POST. The XT/370 has 256 KB on the system board. In XT mode the PC/370-M card supplies the rest of the 640 KB, so POST's memory count and memory test include part of that card.

The first digits of a POST error code identify the subsystem:

POST error code groups relevant to the XT/370

Code	Subsystem
1xx	System board
2xx	Memory
3xx	Keyboard
4xx	Monochrome display
5xx	Colour display
6xx	Diskette drive or adapter
17xx	Fixed disk

The XT keeps its configuration on the system-board DIP switches (SA38 p. 6-3); it has no battery-backed CMOS memory. If a memory error appears and the diagnostics do not isolate it, IBM's method is to reduce memory one bank at a time with the configuration switches until the error goes, then swap modules in the last bank removed.

Dead machine

IBM's dead-machine checks for 5150 and 5160-based systems:

1. Check that the outlet has power and that the power cord is good.
2. Disconnect all external I/O cables.
3. Remove the maths coprocessor if one is fitted.
4. Unplug the drive power connectors one at a time, since a failing drive logic board can load the supply.
5. Measure the voltages from the supply at the system-board connectors.
6. Check the resistance of the system board.

A shorted tantalum capacitor overloads the supply so that it will not run, although its fan may still turn. On IBM 51xx boards and cards these tantalums are usually 10 µF 16 V, and a failed one often shows no visible damage. See the IBM PC XT/370 Capacitor Replacement Guide.

Power supply checks

The 5160 supply is a 130 W switching supply giving +5 V at 15 A (±5 %), +12 V at 4.2 A (±5 %), −5 V at 300 mA (±10 %) and −12 V at 250 mA (±10 %). It shuts down while an overcurrent or overvoltage condition exists.

Measure at the two system-board power connectors with the supply connected and the machine switched on. IBM numbers the twelve pins 1 to 12, P8 first: pin 1 is Power Good, pin 3 +12 V, pin 4 −12 V, pins 5 to 8 ground, pin 9 −5 V and pins 10 to 12 +5 V.

IBM 5150/5160 supply limits at the system-board connectors (SA38 Figure 4-5)

Rail	Meter reading	− lead on pin	+ lead on pin
Power Good	+2.4 to +5.2 V	5	1
+5 V	+4.8 to +5.25 V	5	10
−5 V	+4.6 to +5.5 V	9	6
+12 V	+11.5 to +12.6 V	7	3
−12 V	+10.9 to +13.2 V	4	8

The −5 V and −12 V rails are read with the leads reversed, which is why the table gives positive readings for them. A supply that is not connected to the system board or the drives cannot be measured correctly unless it is loaded with a 1.5 Ω 25 W resistor across P9 pins 1 and 6. IBM's part is a 3 Ω 25 W wire-wound resistor, P/N 0337213; two in parallel give 1.5 Ω.

The mains suppression capacitor in the supply is another common failure. RIFA-made parts rupture with smoke and a strong foul smell, and may leak fluid that should not be touched.

370 card faults

IBM's list of what POST checks (system board, memory expansion adapters, keyboard, primary display, diskette and fixed disk) does not include the 370 processor. Faults on the 370 side show up when VM/PC starts or while it is running. The XT/370 needs PC DOS 2.10 or equivalent, and VM/PC is started from DOS with the VMPC command.

VM/PC startup failures

When VM/PC detects a hardware or software failure, the Session Selection Menu marks the Local 3270 Session as failed. Selecting that session shows "Local 3270 Session Terminated (or Initialization Failure)" with an error message and the prompt to press F1 to retry. A successful retry restarts the local session without affecting host sessions.

VM/PC startup messages (VM/PC User's Guide pp. 4-37 to 4-40)

Message	Meaning	IBM's action
STORAGE ERROR, RC = 2670	Storage error detected	Press F1 to retry. If the retry works, the fault may be intermittent; note the RC. If it fails, run the diagnostics in the XT/370 Guide to Operations.
PROCESSING UNIT ERROR, RC = 2680	Processing unit error detected	As for STORAGE ERROR
HARDWARE ERROR, RC = 2681	Undetermined hardware error	As for STORAGE ERROR
'nnnnnnn' File Not Found, RC = 2695	A file needed for startup is missing	Reload it from the distribution diskette
'nnnnnnn' File Bad, RC = 2696	A file needed for startup cannot be read	Reload it from the distribution diskette
VMPC Configuration file not found	CONFIG.1VM is not in the DOS directory	Run VMPCCON again
VMPC CP I/O Services file not found	CPIO.1VM is missing	Reinstall CPIO.1VM or all of VM/PC
No space under 256K for CP I/O Services	No room in the 256 KB of system-board RAM to load the CP I/O service	No separate action given
Memory block extension failure; VMPC system too large for memory	DOS could not give VM/PC enough memory	Restart with Ctrl-Alt-Del and try again

Failures while running

VM/PC reports hardware and software failures during processing with messages listed in chapter 8 of the User's Guide. IRRECOVERABLE MACHINE CHECK (DMK008T) means the 370 processor has met an error with no recovery. VM/PC shows messages without their numbers unless SET EMSG ON has been issued; IBM suggests turning numbers on when seeking service help.

If a local session stops responding, press SELECT. If the screen does not change, IBM places the fault in the Personal Computer hardware. If the Session Selection Menu appears, IBM places it in the 370 hardware. In both cases the next step is the Personal Computer diagnostics.

Card checks

1. Confirm the cards are in IBM's slots: PC/370-P in slot 4, PC/370-M in slot 3.
2. Reseat both cards.
3. Run the XT/370 diagnostics from the Diagnostics/370 Personality diskette.

Host connection

The XT/370 reaches a host through a 3270 emulation adapter on coaxial cable to the control unit. What terminal it emulates depends on the adapter:

• The PC/3277-EM (3277-2 Device Emulator Adapter), found only on older XT/370s, emulates a 3277-2. It needs the VMPC /7 option to start a 3277-2 session.
• The 3278/79 Emulation Adapter emulates a 3278-2 with a monochrome display, or a 3279-S2A with a Color/Graphics Display Adapter.
• IBM also lists a Remote Display Terminal Device Emulation Adapter, which can sit in slot 2.

While a session is waiting on the computer it is connected to, the X indicator in position 9 of the 25th line comes on. It should go off within one to three seconds of pressing ENTER. If it stays on, press RESET first if INPUT INHIBITED is showing. In a host session, check with the host operator that the host is running, then press SELECT. If the screen does not change, the fault is in the Personal Computer hardware or VM/PC; if the Session Selection Menu appears, the fault is in the connection to the host.

Moving files between DOS and CMS

VM/PC's IMPORT and EXPORT commands copy files between PC DOS and CMS. The ASCII option converts between ASCII and EBCDIC, and EOL uses end-of-line characters to mark record boundaries. Records longer than 80 bytes need the LRECL option.

Related pages

• IBM PC XT/370
• IBM PC XT/370 Maintenance Guide
• IBM PC XT/370 Capacitor Replacement Guide
• IBM PC XT Troubleshooting Guide
• IBM 3270 PC Troubleshooting Guide
• Capacitor Failure Symptoms