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Add the Am7990 Technical Manual and Am79C90 datasheet to reference/lance-datasheet/ and a report, reference/lance-loopback-fcs.md, that answers whether MAME's am79c90 receive() FCS condition (!(MODE_LOOP) || MODE_DTCR) is inverted. Verdict: MAME is right. The CRC generator and checker share one circuit, so with LOOP=1 the receiver can only check the FCS when DTCR=1 -- exactly what MAME does. The proposed inversion would be wrong in both loopback cases. No MAME change made.
LANCE datasheets (Am7990 / Am79C90)
Downloaded 2026-10-16 for the "loopback FCS" question
(../lance-loopback-fcs.md). Originals are preserved unmodified; *.txt are
pdftotext -layout extractions made here. Nothing was edited.
| file | source | notes |
|---|---|---|
Am7990-TechnicalManual-1986.pdf |
bitsavers components/amd/Am7990/1986_Am7990_Local_Area_Network_Controller_LANCE_Technical_Manual.pdf |
primary. Searchable text. §1.9.3 mode register (DTCR p. 1-24), §1.8.3 CRC (p. 1-12), §4.3.3 (p. 4-4). |
Am7990-ReferenceGuide-Aug1986.pdf |
bitsavers components/amd/Am7990/08496A_Am7990_Reference_Guide_Aug1986.pdf |
supporting; no DTCR/LOOP detail beyond the above. |
Am79C90-bitsavers.pdf |
bitsavers components/amd/Am7990/Am79c90.pdf |
primary for the C-LANCE. DTCR bit p. 24, CRC requirements p. 35. Searchable text. |
Am79C90-theretroweb.pdf |
theretroweb chip/documentation/am79c90-66f085c9c1098806690232.pdf |
independent copy of the same datasheet; same page mapping. |
Am7990-bitsavers.pdf |
bitsavers components/amd/Am7990/Am7990.pdf |
standalone datasheet scan without OCR (0 bytes of text). Kept for completeness. |
Am7990-ardent.pdf |
ardent-tool.com/datasheets/AMD_Am7990.pdf |
alternative standalone datasheet scan without OCR. |
*.txt are the extractions of the two searchable PDFs.
SHA-256
bef9659c8c7fcae50630ea779616b01c6b34147125e3a6239300d18409c0d45b Am7990-ardent.pdf
c625457bce8c6b88edba18cc131279730e32d10359dbc3c242a687263b4b1857 Am7990-bitsavers.pdf
d3dfb8a675370068397cc233420db01d0a80e4e5545d6d4eb580e5a218c6455c Am7990-ReferenceGuide-Aug1986.pdf
01be92c5c7a9e90b8319f46f9fed52b446a2676e9b1c362d10e4214b8f9cef72 Am7990-TechnicalManual-1986.pdf
1dbd36baa54ccea3efde4ef8cddb320d07ed84760b4cc547ef804ffe03a76f40 Am79C90-bitsavers.pdf
dd54b5a71cfe59e8ebebcd274b0f06c14ac7591183409f775b356a44bd8fd370 Am79C90-theretroweb.pdf
Key locations (printed page / PDF page)
Am7990 Technical Manual 1986:
- Mode register, bit
03 DTCR/ bit02 LOOP— printed 1-24, PDF p. 31 CYCLIC REDUNDANCY CHECK (CRC)four requirements — printed 1-12, PDF p. 194.3.3 CRC GENERATOR/CHECKER LOGIC TEST— printed 4-4, PDF p. 51
Am79C90 datasheet:
- Mode register, bit
03 DTCR— printed 24, PDF p. 24 Cyclic Redundancy Check (CRC)four requirements — printed 35, PDF p. 35