modem7
10k Member
The ROMs on the IBM 5150 motherboard are MK36xxx types (8 KB in size).
If you desire (or need) to fit a 27256 EPROM (or 27C256) instead of a MK36xxx, you can use the following adapter.
With this adapter, only the first 8 KB in the 27256 will be read by the 5150 motherboard.
On the 27256, by tying VPP to VCC, and tying /CE to ground, the 27256 is being placed in READ mode. In READ mode, the /OE pin is used to gate the data out, and that is equivalent to the U33 /CE pin.
The following picture shows the adapter that I recently constructed.
If you desire (or need) to fit a 27256 EPROM (or 27C256) instead of a MK36xxx, you can use the following adapter.
With this adapter, only the first 8 KB in the 27256 will be read by the 5150 motherboard.
Code:
U33
SOCKET 27256
+----------+ +----------+
| | | |
| 1 (A7) |---------------->| 3 (A7) |
| 2 (A6) |---------------->| 4 (A6) |
| 3 (A5) |---------------->| 5 (A5) |
| 4 (A4) |---------------->| 6 (A4) |
| 5 (A3) |---------------->| 7 (A3) |
| 6 (A2) |---------------->| 8 (A2) |
| 7 (A1) |---------------->| 9 (A1) |
| 8 (A0) |---------------->| 10 (A0) |
| | | |
| 9 (D0) |<----------------| 11 (D0) |
| 10 (D1) |<----------------| 12 (D1) |
| 11 (D2) |<----------------| 13 (D2) |
| | | |
| 12 (GND) |-----------+-----| 14 (GND) |
| | +---->| 20 (/CE) |
| | +---->| 26 (A13) |
| | +---->| 27 (A14) |
| | | |
| 13 (D3) |<----------------| 15 (D3) |
| 14 (D4) |<----------------| 16 (D4) |
| 15 (D5) |<----------------| 17 (D5) |
| 16 (D6) |<----------------| 18 (D6) |
| 17 (D7) |<----------------| 19 (D7) |
| | | |
| 18 (A11) |---------------->| 23 (A11) |
| 19 (A10) |---------------->| 21 (A10) |
| | | |
| 20 (/CE) |---------------->| 22 (/OE) |
| | | |
| 21 (A12) |---------------->| 2 (A12) |
| 22 (A9) |---------------->| 24 (A9) |
| 23 (A8) |---------------->| 25 (A8) |
| | | |
| 24 (VCC) |-----------+-----| 28 (VCC) |
| | | | |
| | +-----| 1 (VPP) |
| | | |
+----------+ +----------+
On the 27256, by tying VPP to VCC, and tying /CE to ground, the 27256 is being placed in READ mode. In READ mode, the /OE pin is used to gate the data out, and that is equivalent to the U33 /CE pin.
The following picture shows the adapter that I recently constructed.