CHA256
C ROMs
Up to 8 * 32Mbit ROMs (27C322) can be used for a maximum of 256Mbit. 27C800/27C160 can also be used depending on jumper configuration.
The size of largest ROM is selected using a pair of jumpers. The rest should be left unconnected.
| Largest C ROM | Jumper set |
|---|---|
| 8Mbit | J5+J6 |
| 16Mbit | J7+J8 |
| 32Mbit | J9+J10 |
A NEO-273 and a LS74 are used to address C ROM. The LS74 provides the 2 most significant bits. The LS139 enables one of four pairs of C ROMs using 2 address inputs depending on which 2 jumpers are connected.
| J5 | J7 | J9 | ? |
|---|---|---|---|
| ROM number LSB is A19 | ROM number LSB is A20 | ROM number LSB is LS74 Q2 (A21 ?) | ? |
| J6 | J8 | J10 | ? |
|---|---|---|---|
| ROM number MSB is A20 | ROM number MSB is LS74 Q2 (A21 ?) | ROM number MSB is LS74 Q1 (A22 ?) | ? |