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) [P] [P]

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

IBM 256KB Memory Card (#6030) [P] [P]

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

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

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

PinSignal NameDir PinSignal NameDir
A01Address/Data Bit 0I/O B01Address Bit 10O
A02Address/Data Bit 1I/O B02Ground
A03Address/Data Bit 2I/O B03Address Bit 11O
A04+5Vdc B04Ground
A05Address/Data Bit 3I/O B05Address Bit 12O
A06Address/Data Bit 4I/O B06Ground
A07Address/Data Bit 5I/O B07Address Bit 13O
A08+5Vdc B08Ground
A09Address/Data Bit 6I/O B09Address Bit 14O
A10Address/Data Bit 7I/O B10Ground
A11Address Bit 8O B11Address Bit 15O
A12+5Vdc B12Ground
A13Address Bit 9O B13Address Bit 16O
A14Address Latch EnableO B14Ground
A15Reserved? B15-Memory WriteO
A16+5Vdc B16-Data EnableO
A17-Memory Card Select 0O B17-Memory Card Select 1O
A18-Memory Card Select 2O B18Ground
A19-Memory Card Select 3O B19RAM EnableO
A20-Memory Card Select 4O B20Ground

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.

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Louis Ohland, Peter Wendt, David Beem, William Walsh, Tatsuo Sunagawa, Jim Shorney, Tim Clarke, Kevin Bowling, Tomáš Slavotínek, and many others.

Ardent Tool of Capitalism - MAD Edition! is maintained by Tomáš Slavotínek.
Last update: 28 Sep 2022 - Changelog | Legal Info & Contact