archive:computers:cart
_A_b_s_t_r_a_c_t
The Atari ST's 128 Kbyte read-only cartridge port can be transformed into a 64 Kbyte read/write port by using this cir- cuit. Writing to this port is slower than reading it by a factor of two, approximately. It can be written to at a speed of 76 Kilowords per second.
This interface can be built by using only three chips, two AS-TTLs and a PAL -- a total parts cost of less than $15.
_H_o_w__I_t__w_o_r_k_s
1. _B_a_c_k_g_r_o_u_n_d__I_n_f_o_r_m_a_t_i_o_n
The ST brings out 15 address lines (A1-A15), 16 data lines (d0- D15) /LDS,[*] /UDS, /AS, /ROM4 and /ROM3.
Of these signals, /UDS, /LDS, and /AS have the same meaning as they as they do for a 68000 CPU. A1-A15 are the least signifi- cant 15 address lines. D0-D15 are connected internally to the ST's data-bus. /ROM3 and /ROM4 are generated by the MMU and are output signals which tell the external hardware which of the two banks is being addressed. /ROM4 is activated when the 68000 tries to read address 0xfa0000-0xfaffff. /ROM3 is activated when the 68000 tries to read address 0xfb0000-0xfbffff. (Note that the names of these signals are counterintuitive; ie. they have been correctly written above.)
2. _C_a_v_e_a_t_s
Any and all hardware connected to this modified port must be fast -- fast enough to run with no wait-states and fast enough to tolerate the decoding delays introduced by this circuit. I've used 200ns static RAM chips without any trouble.
It is advisable to use /ROM4 to time any external hardware you build (in preference to /AS), since /ROM4 contains the neces- sary delay to ensure the addresses are stable when it is activated. (At least this is what I assumed, and it worked.)
The data for write operations is produced by the two AS-TTL latches[*] used in the circuit (see below.) ________ [*] A preceeding slash indicates an active-low signal
Cartridge Port Interface 1 April 14, 1987
3. _O_p_e_r_a_t_i_n_g__P_r_i_n_c_i_p_l_e
i. The PAL contains an asynchronous finite state machine. When
the upper bank (address 0xfbxxxx) is read, the PAL manufac-
tures an A0 (based on the state of /UDS and /LDS) and sup-
plies this to U1. At the end of the read cycle, U1 and U2
are clocked to latch the least significant 16 address bits
into the PAL.
ii. Subsequently, when the lower bank is read (address
0xfaxxxx), the latched address (which was latched in step 1)
is supplied as data and /PWE (the write-enable signal) is
activated (it goes low). /ROM4 active should tell the exter-
nal hardware that it is being addressed, and /PWE active
should tell it that this is a write-cycle. As an interest-
ing side-effect of this design, the data word written out
is read back by the 68000 (you can use this fact to debug
the interface!)
iii. Reads to the upper bank will return garbage unless some ROM
is present there. Multiple successive reads to this bank
will not be harmful (ie. they will not confuse the state-
machine) and the latest address will be latched preparation
for a read to the lower bank.
Note that if you put RAM in the upper-bank you will not
be able to write to it. Connecting the /PWE line to such RAM
will have no effect. Putting a ROM in this address space
should not create any problems (although I haven't tried
this).
Example:
To write data 0xcd01 to address 0x3bc0 in the lower bank, the following steps are performed.
step1. read byte at address 0xfbcd01 (0xfb0000+data)
step2. read word to address 0xfa3bc0 (0xfa0000+addr)
_________________________________________________________________
[*] I used AS374s, you can use AS or FAST. High speed chips are
necessary so that delays from /OE (latch output-enable) to
valid data are not critical. You could possibly get away with
ALSTTL, but I haven't tried this.
Cartridge Port Interface 2 April 14, 1987
_C_o_n_s_t_r_u_c_t_i_o_n
The schematic should help in this department. Constructing this gadget is easy and cheap. The hardest part might be trying to find a connector for this port. You can get one from
Douglas Electronics,
718 Marina Blvd.,
San Leandro, Calif
94577
1-415-483-8770
(part number 33-DE-40. $10.00 per piece)[*]
Another problem you might face is getting the PAL programmed. If you don't know of a way to get the PAL programmed, you could use discrete logic chips to build the state machine.
4. _B_u_i_l_d_i_n_g__i_t
The pin-outs given in the User's guide are CORRECT. The pin-outs given in the "Internals" book are WRONG. Follow the schematic. You can get the pin-outs for the '374s from an ASTTL or FAST databook. The PAL16L8's pin-outs are given below.
________
[*] You guessed it -- I'm not connected with D.E. although I and
a number of others have purchased these connectors from them.
Cartridge Port Interface 3 April 14, 1987
PAL PINOUTS
Legend:
A preceding slash implies an active-low signal.
NC implies no-contact (nothing should be connected to these pins.)
(i) indicates that the signal is an input.
(o) indicates that the signal is an output.
(A) implies that the signal comes from/goes to the Atari cart. port.
(u1) implies that the signal goes to U1, the least-significant
latch.
(u2) implies that the signal goes to U2, the most-significant
latch.
(ex) implies that the signal goes to any external hardware connected
such as a ram-disk.
pin # signal pin # signal ----- ------ ----- ------
1 /UDS (i, A) 11 NC (no contact) 2 /LDS (i, A) 12 /OE (o, U1, U2) 3 /ROM4 (i, A) 13 A0 (o, U1) 4 /ROM3 (i, A) 14 /PWE (o, ex) 5 NC 15 NC 6 NC 16 /dclk (o, U1,U2) 7 NC 17 NC 8 NC 18 NC 9 NC 19 NC 10 gnd (A) 20 Vcc (A)
Note that the line called "help" is shown as
"NC" since it is to be left unconnected.
ALL THE BEST!!
Anees Munshi. April 14, 1986.
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Cartridge Port Interface 4 April 14, 1987
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