Editing IBM PC XT/370 Maintenance Guide (section)

== The Three 370PC Cards ==

The cards must be removed and re-inserted as a '''matched set'''. They communicate over a proprietary card-to-card backplane connector on the rear edge of the cards, in addition to the standard ISA bus.

=== 370PC-P (Processor Card) ===

* Carries the two modified Motorola 68000s ('''Micro/370''') and the modified Intel 8087.
* Heatsink-mounted on the 68000 packages — verify thermal paste integrity at each service interval.
* '''Card cannot be repaired''' if a Micro/370 or modified 8087 fails — these are custom IBM/Motorola/Intel parts.

=== 370PC-M (Memory Card) ===

* Carries 512 KB dual-ported DRAM.
* Communicates with the 370PC-P via the dedicated rear-edge connector.
* Adds 384 KB to the XT host's 256 KB to give 640 KB total host RAM.
* '''DRAMs on the M card''' are standard 64K × 1 DRAM packages of the era; failed DRAMs are replaceable if the failure is identified to a specific chip.

=== PC3277-EM (3270 Emulation Adapter) ===

* '''Earlier-production XT/370s''' carry the dedicated PC3277-EM card.
* '''Later-production XT/370s''' replaced it with the standard 3278/79 Emulation Adapter (same card as on the [[IBM 3270 PC]]).<ref>SA38-0037-00 §6-17.</ref>
* Connects to the IBM 3174 or 3274 cluster controller via BNC twinax.