Ardent Tool of Capitalism

5140 Memory

5140 Memory
Maxing Out the Memory
Memory Cards
   IBM 128KB Memory Card
   IBM 256KB Memory Card
   STB 384KB Memory Card
System Board RAM Connector Pinout

5140 Memory

All RAM is static and does not require refresh. RAM does not use parity. The RAM is contained on cards; each card has 128 KB (or more) of storage. A maximum of four cards can be installed in the system (without using a cable).

Maxing Out the Memory (from Don Hills, edited)

System memory was 640 KB max, with 256 KB as well as 128 KB cards. Any combination could be used up to 640 KB. To reach 640K with 128K cards, several people built a short ribbon cable to extend the last card's end connector to another 128 KB card which they laid on top of the others under the keyboard.

That was too tight for my liking, so I actually ran the ribbon cable through to the modem slot and put the 5th card there.

640K is a hardware limit. Any addresses above 640 KB weren't passed to the memory cards, they were placed on the bus instead for devices like the CGA video memory.

Memory Cards

IBM 128KB Memory Card (#4005)

This card uses 16x SRM2064M-15 chips for 128K total.

IBM 256KB Memory Card (#6030)

This card uses 8x SRM20256M-12 chips for 256K total.

STB 384KB Memory Card "C-RAM 384 (TM)"

U1 74HC139 U2 "C-RAM" PAL? U3 74HC373 U4 74HC245 U5-16 32Kx8 SRAM

U5-16 12x 32Kx8 SRAM - KM62256ALP-10, NCR 26C256L-10, or compatible for 384K total.

No components on the solder side, only rubber feet to support the board.

System Board RAM Connector Pinout

Address/Data Bit 0 – 7

These multiplexed lines are used to form either the low-order bits of an address or a byte of data. At the falling edge of the Address Latch Enable signal, the RAM cards access these eight lines along with lines Address Bit 8 through Address Bit 16 to form an address. At the low level of either Memory Read or Memory Write, the RAM cards access these eight lines to form a data byte. The least significant bit is Address/Data Bit 0.

Address Bit 8 – 16

These lines are used to address memory and I/O devices within the system. The data on these lines is valid through the entire bus cycle.

Memory Card Select 0 – 4

These lines are used to select the card to be accessed. These lines are shifted at the output of the RAM cards.

Address Latch Enable

This line is used to indicate that the address/data bus contains a valid address.

Memory Write

This line is used to instruct a RAM card to store the data present on the data bus.

Data Enable

This line indicates when data should be gated onto the multiplexed address/data bus.

RAM Enable

This line enables the RAM card to be accessed. When this line is low, all other signals to the card are ignored.